GASTRULATION OF THE VERTEBRATES. 415 
Weare in no way justified in including the whole process of 
the formation of the notochord in the gastrulation phenomenon, 
nor either that of the ccelom and the somites. In Echino- 
derms the formation of enteroccel, hydroceel, etc., is only 
inaugurated after gastrulation has been completed; in worms 
the appearance of the somites is synchronic with the increase 
in length which has been noted above and which follows 
upon gastrulation. How the process of ccelomogenesis in the 
higher vertebrates is derived from that of their invertebrate 
ancestors (which are unknown to us) cannot for the present 
be said to be sufficiently elucidated. Certain peculiarities in 
the development of Balanoglossus should here be considered, 
o 56 ES & 
34 @ Yr y c 
0 g 
0 
0 
0 0 
0 0 
Q 0 
s Qa 
Ss a Be 
Ssco? 
Fic. 6.—Gastrula stage of a Mammal. 7, Trophoblast. Zk, Ectoderm. 
En, Entoderm. 
and may to some extent be comparable with what occurs in 
Vertebrates, but up to now we have not sufficient data. 
Bateson has shown that Balanoglossus possesses an unpaired 
anterior ccelomic sac (cavity of the gland), that paired sacs 
of collar-ccelom follow upon this and that an unpaired hinder- 
most ccelom provides the trunk and soon becomes paired.! 
To explain the fact that the mutual relation between the 
earlier gastrulation phases of vertebrates and their later pro- 
} Whether these arrangements are repeated in higher Vertebrates has not 
yet been decisively shown. It should, however, be noticed that the mode of 
formation of the pericardium which was two years ago described by me for 
Tarsius (|.c., p. 3, Pl. VIII, fig. 70 a, 4), is in no way isolated, but has since been 
also observed by me in Sciurus and Tupaja. For Tupaja the gradual transi- 
tion of an evagination of the entoderm that becomes constricted off into the 
VoL, 49, PART 2.—NEW SERIES. 29 
