392 EYCLESHYMER. [Vol. X. 



garded by the author as evidence of concrescence. Considered 

 in the light of the above facts, this view receives confirma- 

 tion. 



Owing to the enormously increased yolk, the germ-ring is cut 

 off much earlier than in the forms represented in Fig. 3, having 

 passed scarcely beyond the region of the equator. 



With the increment of yolk there is a proportional increase 

 in the extent of the primitive streak, until a certain point is 

 reached, when it suffers reduction through the cutting off of 

 the posterior portion of the embryo ; so that in the Chick the 

 anterior portion only of the primitive streak is present, the 

 posterior portion being represented by the line extending from 

 the tail-end of the embryo {d.) to the marginal notch {m.n.), 

 plus the part represented by the rim of the blastopore. In the 

 discussion of homologies it is all-important that this conception 

 be kept in mind. 



From a comparison of the above forms we observe that in 

 Amphibia the greater portion of the embryo is formed through 

 differentiation in situ and overgrowth, concrescence being con- 

 fined to a limited region. 



With the increase in yolk (most Teleosts) a greater extent 

 {p.s.) of the embryo is formed by concrescence, the part arising 

 through differentiation in situ and overgrowth undergoing a 

 reduction. Through a still greater increment of yolk (certain 

 Teleosts, Elasmobranchs, Aves) there is a corresponding in- 

 crease in the extent formed by concrescence, while differentia- 

 tion and overgrowth are of minor importance. 



We conclude that the primitive method of embryo formation 

 in vertebrates is through differentiation in situ and overgrowth, 

 concrescence being a secondary method of uniting the lips of 

 an elongated blastopore, this elongation having arisen through 

 an increase of yolk. 



The general remarks on the position and growth of the em- 

 bryo, especially the Telostean and Avian, are largely the direct 

 outcome of views expressed by Professor Whitman in his un- 

 published lectures on vertebrate embryology, and I gladly 

 acknowledge my indebtedness to him. 



