Blastoderm

“ There is one point to be noticed in the shape of the blastoderm as a whole. It is unsymmetrical, and a much larger number of its cells are found collected at one end than at the other. This absence of symmetry is found in all sections which are cut parallel to the long axis of the egg-capsule. The thicker end is the region where the embryo will subsequently appear. ”

“ The portion of the blastoderm, where epiblast and hypoblast are continuous, forms a projecting structure which I have called the embryonic rim. This rim increases rapidly by growing inwards more and more towards the centre of the blastoderm, through the continuous conversion of lower layer cells into columnar hypoblast.
While the embryonic rim is being formed, the segmentation cavity undergoes important changes. In the first place, it receives a floor of lower layer cells, partly from an ingrowth from the two sides, and partly from the formation of cells around the nuclei of the yolk. ”

“ This envelope is called the blastoderm, and together with its contents will form the embryo. We must merely allude to the fact that it has been considered that some of the nuclei forming the blastoderm arise directly from the egg-mass by a process of amalgamation, and if this prove to be correct it may be admitted that some portions of the embryo are not entirely the result of division or segmentation of combined germ and sperm-nuclei. Wheeler states [73] that some of the nuclei formed by the first differentiation go to form the vitellophags scattered throughout the yolk. ”

“ Figure 2b, through the anterior region of the blastoderm, shows a sharply defined ectoderm (ec) which is composed of three or four layers of cells [234] in the median region, while it gradually thins out laterally. Closely underlying this ectoderm is a thin sheet of irregular cells, the entoderm (en) .
Figure 2c is about one fifth of the length of the blastoderm posterior to the preceding and represents approximately the same conditions, except that there is an irregular thickening of the entoderm in the median region (en) . ”

“ Their real nature is still very doubtful, and though some are no doubt true cells, others are perhaps only nutritive masses of yolk. In a surface view the blastoderm, as the segmented germinal disc may [Pg 150] now be called, appears as a circular disc; the central part of which is distinguished from the peripheral by its greater transparency, and forms what is known in the later stages as the area pellucida. The narrow darker ring of blastoderm, outside the area pellucida, is the commencing area opaca.
Fig. 90. Section of the germinal disc of a Fowl during the later stages of segmentation. ”

“ Whatever may be the course of subsequent changes, the first change is the formation of a superficial layer or blastoderm; and by whatever series of transformations the adult structure is reached, it is from the blastoderm that all the organs forming the adult originate. Why this marvellous fact?
Meaning is given to it if we go back to the first stage in which Protozoa, having by repeated fissions formed a cluster, then arranged themselves into a hollow sphere, as do the protophytes forming a Volvox. ”

“ It appears that the polar body is here preserved for an exceptional period, and its presence can still be proved when the blastoderm has been formed, and sometimes when development is even further advanced. ”

“ Dohrn observed in the larva of ants the rudiments of the ovaries in the form of two pyriform masses, each with eight prolongations which he regarded as young ovarian tubes.
Fig. 552.—Three longitudinal sections through the embryo of Chironomus. In A, the blastoderm (bl) is beginning to form, the polar cells (p) outside of it; in B, the polar cells have pressed in between the blastoderm cells; in C, they lie in the interior of the embryo: b, protoplasmic layer (Keimhautblast) ; d, yolk; k, nucleus of the forming blastoderm.—After Ritter, from Korschelt and Heider. ”