Chap. VI.] GENERAL EMBRYOLOGY. 115 



with a mode of development similar to that which was there 

 described. 



Let us now summarise the more important events in the 

 course of development : 



1. The immature ovum consists of naked protoplasm contain- 

 ing a germinal vesicle (nucleus) and germinal spots (nucleoli). 



2. The mature ovum contains a variable amount of food-yolk, 

 but little in Amphioxus, and that little uniformly distributed 

 (alecithal) ; more or less in fowl and frog, concentrated at one 

 pole (telolecithal) ; but little in the rabbit (alecithal). 



3. The detached ovum is surrounded by primary and 

 secondary membranes. The primary membranes (zona radiata, 

 rabbit ; vitelline membrane, fowl and frog) are formed in the 

 ovi-sac. The secondary membranes (albuminous layer, shell 

 membranes) are added in the oviduct. 



4. The nucleus, previous to fertilisation, divides twice, pro- 

 bably by karyokinesis ; one half is extruded in each case in a 

 polar cell ; the portion that remains is the female pronucleus. 



5. Fertilisation is probably effected by a single spermatozoon, 

 the nucleus of which, within the ovum, forms the male pro- 

 nucleus. Fertilisation is effected after the ova are laid in the 

 frog, but within the body of the parent fowl and rabbit. 



6. Female and male pronucleus fuse to form the first segmen- 

 tation nucleus. 



7. Segmentation is probably in all cases accompanied by 

 karyokinesis. 



In Amphioxus, holoblastic and nearly regular. 



In frog, holoblastic and unequal. 



In fowl, meroblastic and irregular. 



In rabbit, holoblastic and slightly unequal. 



8. Epiblast and hypoblast produced 



In Amphioxus by simple imagination. 



In frog partly by invagination, partly by differentiation. 



In fowl and rabbit by differentiation. 



9. Archenteron produced 



In Amphioxus by invagination. 

 In frog partly by invagination. 



In fowl and rabbit beneath the blastoderm of the 

 embryonic area. 



