'25S 



K. MITöUKUßl. 



Woodcut IV. 



yoJk in the Elaismobranch egg. If we make a companion diagram 

 to the well-known series given by Ballour (Comp. Embryol. vol. II. 



Fig. 175) it will be like Woodcut 

 IV. The upper pole of the 

 egg in Keptilia is capped 

 by a small patch where 

 nearly all the changes 

 which in Amphibia are 

 gone through by the whole 

 Qgg, are performed. Ac- 

 cordingly, the enclosure of the 

 yolk by the blastoderm in the 

 chelonian egg is of a very difterent 

 nature from the enclosure of the 

 yolk in the Elasmobranch, for while the former is a simple growth of 

 the edge of the l)lastoderm, and of cœnogenetic character, the latter is a 

 ]jart of the process of invagination and of palingenetic character. 

 That the yolk in Chelonia is not completely enclosed till the embryo 

 has made much progress is due to its large size, and may be regarded 

 as of quite secondary significance. I thus find myself obliged to put 

 aside tlie yolk-l)l;istopore of ])alfonras no longer tenable in Sauropsida. 

 After what has been given above, I need hardly say that I accept 

 the views of Rabl (No. 9) as to the loss and acquisition of the yolk in 

 vertebrate eggs several times in the course of the phyletic development. 

 All the facts given Jibove tend to pro^e that Chelonia possesses a 

 secondary meroblastic ovum in contrast to the primary meroblastic 

 ovum of the »Selachians. 



My views overlap more or less those of previous writers, such as 

 Wenckebach (No. 15), Will (Nos. 18, 1!) & iM), Mehnert (No. 8) 

 and Rabl (No. *J). It would, however, be a tedious and useless task 



