254 AINMAL BIOLOGY. [Part II. 



5. The protoplasm of the ovum is largely converted into deep 

 brownish-yellow vitellus or yolk. 



6. A vitelline membrane (v. m.) is formed around the ovum 

 with its vitellus (Fig. 79, B.). 



7. The ovisac bursts and the ovum is set free. The vitelline 

 membrane continues to invest it. 



8. In the oviduct it becomes invested with a viscid trans- 

 parent substance which sets into a tougn case. This becomes 

 suspended by a stalk of the same material to the swimmerets of 

 the female. 



The exact manner in which the spermatozoa unite with the 

 ova is not known. 



Development. Owing to the large amount of food-yolk segmen- 

 tation is partial or incomplete. In the fowl, where the segmen- 

 tation is incomplete, it is also irregular, owing to the fact that 

 the germinal disc of formative protoplasm is placed at one pole 

 of the egg (telolecithal). In the crayfish the segmentation is 

 regular, owing to the fact that the formative protoplasm is at 

 the circumference, the food-yolk being central in position 

 (centrolecithal). 



By segmentation, a blastodermic layer (b. I.) is formed (Fig. 

 79, C.) as an investment to the food-yolk, which is arranged in 

 conical masses. It is composed of nucleated protoplasmic plates, 

 but the relation of the nuclei of these plates to the germinal 

 vesicle is not well ascertained. Out of these protoplasmic plates 

 the embryo is fashioned. 



The first change is that the blastodermic layer becomes 

 thickened over an area -^ inch long in the neighbourhood of the 

 pedicle, forming the germinal disc. Invagination takes place at 

 the hinder end of this area (79, D.). Thus we have a differentia- 

 tion of the blastodermic layer into epiblast (ep.) and hypoblast 

 (hy.) with the formation of an archenteron (arch.) and a blasto- 

 pore (II). 



The mesgblast cells make their appearance (m.) in the neigh- 

 bourhood of the blastopore between the hypoblast and the 

 epiblast, its cells being probably derived from the former. 



