332 GENERAL EMBRYOLOGY 



The most highly modified type of Chordate blastula is found 

 in those forms with extremely meroblastic, telolecithal eggs, 

 where cleavage is of the discoid type. This condition is com- 

 mon to the Elasmobranchs (Fig. 158), the true Teleosts (Fig. 

 150, C), and to the higher Craniates the Reptiles, Birds. In 

 reality this is, in a modified way, characteristic of the Mam- 

 mals also, for although the Mammalian ovum is nearly alecithal, 

 it is clearly derived from the Reptilian condition, and many 

 features of its development show unmistakably the effects of a 

 large yolk content previously present, but now lost in correla- 

 tion with the newly acquired source of nourishment possessed 

 by the Mammalian embryo. The result of discoid cleavage is 

 the formation of a small mass of living active cells, the blasto- 

 derm, or blastodisc, or germ disc, lying upon the surface of the 

 yolk mass. The blastula instead of being spherical, has there- 

 fore the form of a circular disc, the cellular elements of which 

 can really be compared, at first, only with the cells of the animal 

 pole of the spherical blastula, the unsegmented yolk represent- 

 ing, in this stage, the large cells of the vegetal pole of such a 

 blastula. In comparing these two types of blastulas we may 

 imagine that the ordinary spherical blastula has been cut in two 

 horizontally, through or just above its equator, and the animal 

 hemisphere flattened out, its circumference being thereby 

 somewhat extended. This form of blastula (discoblastula) is 

 several cells in thickness and is usually separated from the 

 underlying yolk by a shallow space called the sub-germinal 

 cavity which represents the blastocoel (Fig. 150, C). While 

 the yolk mass is usually by no means wholly devoid of nuclei, 

 these may or may not be associated with true cellular structures, 

 and even when present at this stage these rarely give rise to 

 any structural parts of the definitive embryo when this forms. 

 The Mammalian blastula diverges widely from any of these 

 conditions and on account of its very special character further 

 reference to it may be omitted here. 



The next important step in development consists essentially 

 in the conversion of this monodermic blastula into a didermic 

 organism, that is, one in which the cells are arranged in two, 



