600 E. W. MACBRIDE. 



stricted off becoming formed into a somite, and nipped off 

 from the open part of the coelomic groove, which then again 

 grows in length, and the process is repeated. The formation 

 of a somite, then, is essentially a process of obliterating the 

 cavity of the ccelomic groove for a certain space, and the 

 so-called last somite is really the undifferentiated hinder 

 end of the coelomic groove. 



The entire independence of the collar cavities from the 

 ccelomic grooves is emphasised by the fact that for some time 

 after the latter are shut off from the gut the collar cavities 

 still retain their openings into it. This is shown in the trans- 

 verse section (fig. 14, c) and in the longitudinal section (fig. 

 15, a). These collar cavities are the " first protovertebrse " 

 of Kowalevsky, and in his paper (7) he notes the fact that 

 they communicate by a broader slit with the alimentary canal, 

 and retain this communication longer than the rest. Later, 

 it is true, the right collar cavity becomes completely shut off 

 from the gut, but the left retains its communication, as is 

 shown in PI. 43, fig. 16. 



Shortly after this period the embryo begins to diminish 

 rapidly in diameter, owing to the consumption of the yolk in 

 the endoderm cells, whilst at the same time it increases in 

 length, and the cavities in its interior diminish in size, owiug 

 to the gradual shrinkage. Hence one requires now to have 

 specimens preserved in such a way as to give firmness and 

 resistance to the outer tissue, if one is to make out anything 

 of the internal anatomy at all. The yolk, which is present not 

 only in the ventral but also in the dorsal ectoderm and in the 

 walls of the coelomic folds, acts whilst it endures, somewhat 

 like parafiin, in preventing too great shrinkage ; after it has 

 gone nothing but osmic acid will give any help. 



Just as the disappearance of the yolk is commencing, the 

 third division of the mesoderm, the head cavities, make their 

 appearance. The two head cavities really constitute the extreme 

 anterior end of the alimentary canal, which grows out into two 

 lateral horns. In PI. 44, fig. 17, a, we see them still opening 

 widely into the gut; but in the next section (17, b), taken 



