396 SECTIONAL TRANSACTIONS.—D. 
Friday, September 25. 
Discussion on Vertebrate Embryology :— 
Prof. E. W. MacBripg, F.R.8.—The Formation of the Layers in 
Vertebrata. 
The question of the method of the formation of the layers, 7.e. the primary tissues 
ectoderm and endoderm and mesoderm, is of fundamental importance in embryology, 
since if there is a recapitulatory basis for development this method should give some 
hints as to the steps by which a Protozoon was converted into a Metazoon. Amongst 
the higher vertebrata, the early stages are complicated by the influence of yolk, 
uterine development and so on; hence reliance is placed on the development of 
amphibia in which the yolk is of moderate amount and the whole egg segments into 
cells. 
Recently Vogt has developed a method of marking the living eggs of Urodela and 
Anura by pressing against them in the stage when segmentation has just been com- 
pleted small fragments of agar soaked in neutral red, Bismarck brown or Nile blue. 
As the cells which are stained move, these marks are carried withthem. Segmentation, 
as is well known, results in the formation of a blastula or hollow ball, with small cells 
above and large yolky cells below. At a point in the midst of the yolky cells a small 
pit appears ; this is the beginning of invagination, which is an active process of inflow; 
the initial pit is subsequently found at the extreme anterior end of the gut. The 
inflow involves not only the large yolky cells which become converted into the 
endodermic epithelium, but at the dorsal lip of the opening and at its sides small- 
celled material ; the dorsal material gives rise to the notochord and the lateral to the 
mesoderm. The mesoderm does not, as Hertwig supposed, originate from the dorso- 
lateral wall of the gut, but grows in from the edges of the blastopore and tends forwards 
and upwards. After the endoderm has been invaginated there is an active creep or 
‘ epiboly’ of the blastoporal rims over the hinder end of the yolk (the yolk-plug) so 
that the blastopore is eventually closed, leaving in Urodela a small residual opening 
whichistheanus. The blastopore in its later stages becomes a vertical slit ; the nerve- 
cord is not lengthened by the meeting of the edges of this slit, but these edges give 
rise to the tail bud. 
Vogt is inclined to agree with Lwoff in regarding all the small-celled material as 
ectoderm; this is equivalent to saying that ectoderm and endoderm are sharply 
differentiated before invagination takes place, and that the mesoderm and notochord 
are ectodermic invaginations. There are grave, and, it seems to us, decisive objections 
to this point of view: (1) not all the mesoderm grows in from the edge of the blastopore— 
there is in Elasmobranchs an anterior vesicle cut off from the front end of the gut. 
which gives rise to the eye-muscles ; (2) the development of the simplest known egg, 
that of Amphioxus, which has a minimal amount of yolk, gives no-countenance to 
Vogt’s views. In Amphioxus all the nuclei of the blastula have vesicular nuclei 
which absorb little stain and all the nuclei of the invaginated cells, those which will 
eventually form the notochord as well as those destined to form the gut epithelium, 
retain this type of nucleus. On the other hand, the nuclei of the outer cells which 
form the ectoderm and the nerve-cord become dense and absorb much stain. The 
limits of the two kinds of nuclei can be seen sharply defined at the edges of the 
blastopore. The mesoderm arises from five hollow pouches of the gut, one median, 
ventral and anterior which corresponds to the pouch which in Elasmobranchs gives 
rise to the eye-muscles, and two dorso-lateral pairs close behind it. In the short, 
thick, early embryo the openings of the two pairs are close to each other ; the openings 
of the hinder pair being well in front of the blastopore. The front pair correspond 
to the mesoderm, which in Elasmobranchs gives rise to the superior oblique eye- 
muscles ; the hinder pair correspond to the ‘ mesodermic bands ’ of higher vertebrata 
which become metamerically segmented into myotomes. As the embryo grows in 
length the openings of this hinder pair of pouches become carried back to the hinder 
end of the embryo, whilst pari passu the front ends of these pouches become segmented 
into myotomes. These pouches are well represented in the embryo of Balanoglossus 
and in the Tornaria larva : the anterior median one being the rudiment of the proboscis 
cavity, the anterior pair the rudiments of the collar cavities, whilst the posterior pair 
give rise to the trunk coelom. The openings of the posterior pair are near those of 
