FATE OF BLASTOPORE, ETC., IN CBELOXIA. 91 



dcvclopwciit, the ciiijs of the ancestors of the Awniota acriuired for the 

 second time a lanje yolk-nmss, u-ltich I maij call tlie secondarij yolli- 

 iiiass. It is this irhich ire see in the Amniote eggs of the present daij. 

 The enclosure of the primarg ijolk-inass Inj the hlastoderm is closehj 

 connected irith the process of gastrulation and lias a palingenctic cliaracter. 

 The enclosure of the secondarij yolh-iiiass hj the blastoderm is a coenogcnetic 

 process and has nothing to do vith gctstrulation. Thus it becomes 

 imperative to clistingnish the meroblastic e'^g of the Ehismobranchii 

 from the meroblastic eo-g of the Amniota. The former may be 

 called the primarif vicmhlastic orum or proto-merohlastic orvnh and the 

 latter the secondarij merohlastic onun or meta-merohlastic oruui. All 

 comparisons of the two classes of eggs without taking the above 

 circumstances into consideration are apt to lead us astray. 



These conclusions may at first sight appear somewhat fantastic, 

 l)ut if the facts are really as brought out in the present article — and 

 I have taken great pains to test their correctness and am convinced 

 of their sul;)stantial correctness — the conclusions seem to me ine^■i- 

 tal)le. The idea of the loss and acquisition of yolk in the vertebrate 

 eo-o-s several times in the course of phyletic development is no new 

 one of mine. It is Kabl ('89) who first formulated it and discussed 

 it in a most masterly manner in his " Theorie des Mesoderms." 

 While wa-iting Contribution If, the facts then in my possession 

 convinced me of the correctness of the idea, and I discussed the 

 question towards the end of tliat article. The facts brought out 

 since and given in the present contribnti<jn appear to me to leave no 

 choice in the matter, and furnish, to me at least, absolute proofs of the 

 soundness of the theory. 



3. It is remarkal3le that when once the above homology is accept- 

 ed, the difficulties that have always stood in the way of homologising 

 the icthyopsidan with the sauropsidan egg dissolve away. Balfour 



