324 



CLEAVAGE (SEGMENTATION) AND BLASTULATION 



Fig. 160. Cleaving eggs of Platypus and Echidna. (After Flynn and Hill, '39.) (A) 

 Egg, shell, and early cleavage in Ornithorhynchus. (B) Early cleavage in Echidna. See 

 fig. 161D. 



synchronization of mitotic division is lost. In certain other teleost fishes, lati- 

 tudinal cleavages begin as early as the 8-cell stage. 



At the 32- to 64-cell stages in Serranus atrarius, the blastoderm presents a 

 cap-like mass of dividing cells overlying a forming blastocoel (fig. 159H, I). 

 Between the blastocoel and the yolk, there is a thin layer of protoplasm 

 connecting the edges of the cap. This thin protoplasmic layer is the forerunner 

 of the central periblast tissue; at this stage it contains no nuclei (fig. 159F, H). 



2) Origin of the Periblast Tissue in Teleost Fishes. In the sea bass and 

 many other teleost fishes, some of the surrounding cells at the edge of the 

 blastoderm lose their cell boundaries and fuse together to form a common 

 syncytial tissue. The nuclei in this tissue continue to divide (fig. 159J) and 

 eventually migrate into the periblast tissue below the blastocoel (see arrow, 

 fig. 159L). The latter then becomes the central periblast, while the syncytial 

 tissue around the edges of the growing blastodisc forms the peripheral or 

 marginal periblast (fig. 159K-M). 



In the trout, the early cleavage furrows of the blastodisc are incomplete, 

 and the periblast arises from the syncytial tissue established directly below 

 and at the sides of the protoplasmic cap (fig. 159N-R). This condition re- 

 sembles the cleavage process in the elasmobranch fishes. 



See Kerr ('19); Kopsch ('11); and H. V. Wilson (1889). 



d. Prototherian Mammalia 



The Prototheria normally are placed in the class Mammalia along with 

 the Metatheria (marsupials) and Eutheria (true placental mammals). How- 



