THE GERM-CELLS 49 



is formed by the doubling-in of the blastula, thus leading 

 to the formation of the two primitive germinal layers, the 

 outer, or ectoderm, and the inner, or entoderm, both en- 

 closing the primitive mouth-cavity. Finally, the third 

 layer, the mesoderm, is formed between ectoderm and 

 entoderm, and the formation of tissues, organs, etc., goes 

 on apace until the final form of the completed embryo is 

 reached. 



We can distinguish, according to the four kinds of ova, 

 four kinds of segmentations, with four kinds of gastrulse, 

 namely : 



1. The egg with diffuse yolk shows equal segmentation 

 (all the cells dividing evenly) and a bell-shaped gastrula. 

 Types : Sponge, Amphioxus. 



2. The egg with central yolk, the outer portion only 

 dividing, has superficial segmentation, and a spherical 

 gastrula. Types : Crusters, Insects. 



3. The egg with polar yolk has unequal segmentation, 

 the cells at the upper pole, free from yolk, dividing faster 

 and leading to a hooded gastrula. Types : Amphibians. 



4. The ovum with predominant yolk, where again only the 

 germinal vesicle divides on top of the yolk, thus leading to 

 the discoid gastrula. Types: Reptiles, Birds, Monotremata. 



There is only one more point to which we wish draw atten- 

 tion in this connection. As the mammals have descended 

 genetically from the reptiles and birds, yet have lost the 

 yolk of their eggs (the mammalian embryo is nourished 

 within the uterus of the mother, and does not need the 

 yolk), the development of the mammalian ovum has re- 

 verted from the bird-type, which is still prevalent among 

 the Monotremata, the lowest mammals, to a more primitive 

 type, though the exact details are not yet completely 

 cleared up. On the other hand, the lowest vertebral 

 animal, the Lancelet or Amphioxus, has a segmentation 

 of the most primitive type, like the sponge, showing thus 

 the close connection between the two species so far distant 

 in the scale of organization. 



7 



