320 CLEAVAGE (SEGMENTATION) AND BLASTULATION 



A transverse section through one of the diameteis of the primary blastula 

 presents the following features (fig. 156G): 



(a) A central mass of cells of two or several cells in depth overlies the 

 blastocoelic space. This is the central or cellular portion of the 

 blastoderm. 



(b) Underneath this central blastoderm is the primary segmentation cavity 

 or primary blastocoel. 



(c) Below the primary blastocoel is the central syncytial periblast, which 

 continues downward to the yolk material; many yolk granules are 

 present in the layer of the central periblast near the yolk. Nuclei are 

 not present in the central area of the central periblast, but may be 

 present in its more peripheral portions. 



(d) Around the peripheral areas of the central periblast and the cellular 

 portion of the blastoderm is the marginal periblast tissue which now 

 is called the germ wall. The germ-wall tissue contains much yolk ma- 

 terial in the process of digestion and assimilation. 



The central mass of cells or cellular blastoderm increases in cell number 

 and in size by the multiplication of its own cells and by the contribution of 

 marginal periblast tissue which gradually forms cells with boundaries from 

 its substance. The germ wall thus may be divided into two main zones: (1) 

 an inner zone of distinct cells, which are dividing rapidly and, in consequence, 

 contribute cells to the peripheral portions of the growing cellular blastoderm 

 and (2) an outer peripheral zone, the syncytial germ wall (zone of junction). 

 The latter is in intimate contact with the yolk (fig. 156G). The central peri- 

 blast tissue gradually disappears. At the outer boundary of the peripheral 

 periblast, there is an edge of blastodermic cells overlying the yolk. These 

 cells have complete boundaries and are known as the margin of overgrowth 

 (fig. 156G). A resume of the early development of the hen's egg in relation 

 to the parts of the oviduct, pituitary control, laying, etc., is shown in figure 157. 



4) Polyspermy and Fate of the Accessory Sperm Nuclei. The bird's egg 

 is polyspermic and several sperm make their entrance at the time of fertili- 

 zation (see fig. 157B). The supernumerary sperm stimulate abortive cleavage 

 phenomena in the peripheral area of the early blastodisc (fig. 155D). How- 

 ever, these cleavage furrows together with the extra sperm nuclei soon 

 disappear. 



(References: Blount ('09);Lillie ('30);OIsen ('42); and Patterson ('10).) 



For later stages in the development of the hen's egg, see chapter 7. 



b. Elasniobranch Fishes 



1) Cleavage and Formation of the Early Blastula. Like the egg of the bird, 

 the egg of the elasmobranch fishes is strongly telolecithal, and a small disc 

 of protoplasm at one pole of the egg alone takes part in the cleavage phe- 



