292 MESOBLAST AND NOTOCHORD. 



It might have been supposed that this process was equivalent 

 to the growth of the blastoderm round the yolk in the Sauro- 

 psida, but then the blastopore ought to be situated at the pole of 

 the egg opposite to the embryonic area, while, according to Van 

 Beneden, the embryonic area corresponds approximately to the 

 blastopore. 



Van Beneden regards the Mammalian blastopore as equiva- 

 lent to that in the Amphibia, but if the position previously adopt- 

 ed about the primitive streak is to be maintained, Van Bene- 

 den's view must be abandoned. No satisfactory phylogenetic 

 explanation of the Mammalian gastrula by epibole has in my 

 opinion as yet been offered. 



The formation of the blastodermic vesicle may perhaps be 

 explained on the view that in the Proto-mammalia the yolk-sack 

 was large, and that its blood-vessels took the place of the pla- 

 centa of higher forms. On this view a reduction in the bulk of 

 the ovarian ovum might easily have taken place at the same time 

 that the presence of a large yolk-sack was still necessary for the 

 purpose of affording surface of contact with the uterus. 



TJic formation of the Mesoblast and of the Notochord. 



Amphioxus. The mesoblast originates in Amphioxus, as in 

 several primitive invertebrate types, from a pair of lateral 



FIG. i So. SECTIONS OF AN AMPHIOXUS EMBRYO AT THREE STAGES. 



(After Kowalevsky.) 



A. Section at gastrula stage. 



B. Section of an embryo slightly younger than that represented in fig. 169 D. 



C. Section through the anterior part of an embryo at the stage represented in 

 fig. 169 E. 



np. neural plate ; nc. neural canal ; mes. archenteron in A and B, and mesenteron 

 in C; ch. notochord ; so. mesoblastic somite. 



