u6 ANIMAL BIOLOGY. [Parti. 



10. Mesoblast produced 



In Amphioxus from paired outgrowths of the archen- 



teron. 

 In frog partly from hypoblast at point of invagi- 



nation, partly by differentiation of yolk-containing 



cells. 

 In fowl and rabbit partly by down-growth of cells 



derived from the epiblast at primitive streak, partly 



by differentiation from hypoblast. 



11. Neural plate, groove, and tube formed in all four types 

 from epiblast of mid-dorsal line, the process being somewhat 

 modified in Amphioxus. 



12. Notochord formed in all four types by special up-growth of 

 cells of hypoblast in the mid-line beneath the medullary axis. 



13. Mesoblastic plates in all three types separate into verte- 

 bral plates and lateral plates, of which the former by transverse 

 segmentation give rise to mesoblastic somites, and the latter, by 

 cleavage of the mesoblast, to the body-cavity. 



14. The embryonic body is formed in the frog by simple 

 elongation, there being no yolk-sac or umbilical vesicle. In the 

 fowl and rabbit it is folded off from the yolk-sac or from the 

 umbilical vesicle. 



15. In the fowl and rabbit a protective sac, the amnion, is 

 formed over the embryo. This is absent in the frog. 



16. In the fowl and rabbit an embryonic respiratory organ, 

 the allantois, is developed as an outgrowth from the hind-gut. 

 In the rabbit this comes into close relation with the uterine 

 tissues of the mother, and thus acquires also a nutritive func- 

 tion. It is absent or rudimentary in the frog. 



17. Tertiary investing membranes (serous membrane in fowl, 

 serous membrane ,and chorion in rabbit) are produced in the 

 higher types. 



