FA'J'E OF BLASTOPORE, ETC., IX CHELoNIA. 95 



part of the large- celled floor of the archenteron plus the yoUc-pliKj* in 

 the amphibian ovum, after ga.strnlation has taken place (see, foi- 

 instance, lb and d in Fig. 58,** Hertwig, Lehrhucli, 4te. Aufl. p. 89). 

 At later stacres, wlien the ]n-oblematical cell-mass has receded to the 

 edcre of the embryonic area (Fig. S etc.), it represents less, and corres- 

 ponds simply to the yolk-mass ]ieeping through the yolk-blastopore 

 of the Elasmobranchii. What, it corresponds to in the amphibian 

 ovum is somewhat douljtful, as the discussion in the next paragrnph 

 will show. If tlie homology of di Itèrent parts which I have set forth 

 here is true, we should expect a priori that the priinitive plate will 

 give rise to the structures wdiich are formed out of, or at least, in 

 contiguity with, the lower larger-celled half of the amphibian ovum, 

 or its liomologue in Elasmobranchii, while the secondary yolk-mass 

 ought not t<) Ije of much significance in this respect. In point of fact, 

 the facts brouo-ht out in my Contrihutioit IV fall in well with these a 

 priori considerations. The conclusion which I set forth in tli;it 

 Coiitrihutioii as justified by the facts Ijrought out there are as follows : — 

 " From the epihlast of the emhrijonic shield, the epiblast and its deriv- 

 atives of the future animal is derired. In tlie region of the primitive 

 plate and its anterior enlargement are produced the invagination- 

 CAViTY (the Archenterofi), the yolk-plug, the chorda, the meroblast 

 (both peristomcd and gastralf aud the definitive hypoblast and ITS 

 derivatives. The primitive hirer lai/erf forms the wall of the yolh-sac, 

 and contrihutes to tlie future animal onltj in so far as some of its cells are 

 unrccognisahltj incorporated with the cells of the priuiitice l:noh, nhen the 



* The nauie Yolh-riu/i wliich I have a.[iplie<l to the probIeu)atical cell uiass is aecordiugly not 

 entirely happy, as the mass corrcspotids to somethinu- ujore than the yolk-plug-. But as it is the 

 name which I have used from the first Coiitribntioii aud as it answers on the whole my purpose 

 best, I have retained that name in this paper. 



** .-V well-known figure, widely copied, showing the longitudinal section of a Triton egg 

 after the completion of gastrulation. 



t Represented by the dotted line in Woodcut XVf. 



