THEOEY OF THE EMBRYONIC STAGE OP ONTOGENY. 337 



Thus the coelom appears to be^phylogenetically, simply a differ- 

 entiated portion of the archenterou ; when the lumen of the 

 latter is sraall^ and its walls are composed of only a few large 

 cells, the mesodermic walls of the coelom are represented by a 

 single large cell on each side. 



A little consideration will throw light on the reason why 

 what we see to be probably the primitive mode of development 

 should appear in Paludina Brachiopods and Amphioxus. In 

 all these cases the yolk is exceedingly small in quantity and 

 uniformly distributed ; in the first two cases we have a 

 pseudo-embryonic development — the fourth type mentioned 

 above, — and of course in this case there are no pelagic conditions 

 to require a suppression of the coelom. Amphioxus has a long 

 larval history. Sagitta, on the other hand, pursues a true 

 embryonic development within the egg-membrane ; but the 

 yolk appears to be quite uniformly distributed, and hence its 

 primitive character. 



A second vexed question which naturally follows directly on 

 that of the origin of the mesoderm, is the origin of the 

 endoderm, and consequently of the gastrula itself. Echinoderm 

 development suggests the idea that the ancestral form of 

 metazoon was a sphere of ciliated cells, and that the archenteron 

 arose through the specialisation of a portion of the surface of 

 the sphere to fulfil digestive functions and its invagination 

 into the anterior. This is the view adopted by Korschelt 

 and Heider ;^ and it is, of course, the famous gastrsea hypothesis 

 of Haeckel.^ On the other hand, contrary opinions have been 

 put forward by MetschnikoflF,^ Lankester,^ and Sedgwick.^ 



' 'Lehrbuch der Vergleicheude Etnbiyologie,' vol. i, p. 81. Heider 

 showed experimeutallji that carraiue granules were swept by the cilia to the 

 posterior end. 



^ Haeckel, 'Studien der Gastraea Theory,' Jena, 1877. 



^ El. Metschnikoff, " Spongiologische Studien," ' Zeit. fiir wiss. Zool.,' 

 Bd. xxxii, 1879. 



* E. Ray Lankester, " Notes on Embryology and Classification," 'Quart. 

 Journ. Micr, Sci.,' vol. xvii, 1877. 



* A. Sedgwick, " The Development of the Cape Species of Peripatus, 

 pt. iii," ' Quart. Journ. Micr. Sci.,' vol. xxvii, 1887. 



