322 CLEAVAGE (SEGMENTATION) AND BLASTULATION 



is in reptiles and birds. In the latter groups, these accessory nuclei may divide 

 for a time but ultimately degenerate, playing no real part in ontogeny. In the 

 case of the elasmobranch fishes, the accessory nuclei tend to persist somewhat 

 longer, and accordingly, it is upon this evidence that some have maintained 

 that the periblast nuclei arise from them. Others hold that the sperm nuclei 

 degenerate as they do in reptiles and birds, and the periblast nuclei arise as 

 a result of the regular embryonic process. A third view concedes that both 

 these sources contribute nuclei. 



In view of the origin of the periblast nuclei in teleost fishes, in the ganoid 

 fishes, Amia and Lepisosteus, and in reptiles and birds, and of the syncytial 

 tissue of the later mammalian trophoblast, it is probable that embryonic cells 

 and tissues and not accessory sperm nuclei are the progenitors of the periblast 

 tissue. This probability is suggested by figure 158F, G. Furthermore, later on 

 in the development of the elasmobranch fishes, the entoderm appears to con- 

 tribute nuclei which wander into the periblast tissue which lies between the 

 entoderm and the yolk material (fig. 213K, L). In later stages the periblast 

 tissue is referred to as the yolk syncytium. In the yolk syncytium the periblast 

 nuclei gradually assume a much larger size. 



For further details of the early development of the elasmobranch fishes, 

 consult Ziegler ('02) and Kerr ('19) and Chapter 7. 



c. Teleost Fishes 



1) Cleavage and Early Blastuia Formation. During the fertilization process 

 of the egg in teleost fishes, the superficial cytoplasm of the egg migrates toward 

 the point of sperm entrance and hence a mound-like disc of protoplasm forms 

 at the pole of the egg where the sperm enters (figs. 122C; 123B, C). It is 

 this protoplasmic mass which takes part in cleavage (fig. 123E). The cleavage 

 planes in the teleost fishes manifest great regularity. The early cleavage fur- 

 rows almost cut through the entire protoplasmic disc in most teleost eggs, 

 and a mere strand of cytoplasm is left near the yolk which is not cleaved 

 (fig. 159E). 



In the sea bass, Senaniis atrariiis, the first two cleavage planes are merid- 

 ional and at right angles to each other (fig. 159A); the third planes are ver- 

 tical and parallel to the first plane. The result is a group of eight cells in two 

 rows (fig. 159B). The fourth cleavage furrows are vertical and parallel to 

 the long axis of the eight cells previously established. These furrows divide 

 each of the eight blastomeres into inner and outer daughter cells. The result 

 is 16 cells, arranged in parallel rows of four cells each (fig. 159C, D). 



As the 16-cell condition is converted into 32 cells, the four inner cells 

 divide latitudinally, that is, the cleavage spindle forms perpendicular to the 

 surface, while the twelve surrounding cells divide vertically (fig. 159D, F, G). 

 From this time on latitudinal and vertical cleavages become mixed, and the 



