8S 



MARINE BIOLOGICAL LABORATORY. 



the cells of the two layers, but later one layer might 

 specialize for nutritive purposes, and the other for 

 locomotor (Fig. 5). From such a condition, by the 

 gradual bending of the plate so that the nutritive layer 

 becomes concave (Fig. 7), a gastrula could readily be pro- 

 duced, and in this con- 

 nection Biitschli points 

 out that the concavity 

 would be useful to the 

 colony, serving as a trap 

 for food -particles and 

 also allowing a larger 

 number of cells to come 

 into contact with a 

 larger food body. 



The blastula which so 

 frequently precedes in- 

 vagination could be pro- 

 duced from this by the accumulation of fluid between 

 the two layers ; and furthermore, delamination may have 

 been brought about by the plate, while still one-layered, 

 becoming concavo-convex (Figs. 6, 8 and 9), and finally 

 a hollow sphere (Fig. 10), the transverse division of the 

 constituent cells then taking place. 



Unfortunately for this theory it does not find general 

 support in embryological phe'nomena, nor any more 

 than the Planula theory does it explain the formation 

 of the blastula in a manner in accordance with the 

 actual facts. 



There is a series of phenomena which none of these 

 theories attempt to explain. Many of the authors who 

 have recently contributed to our knowledge of Coelen- 



FiGS. 5-10. 



