338 A. Hyatt — Larval Theory of the Origin of Tissue. 



during the gastrula stage, and are supposed to be identical with 

 the differentiated cells often found in the blastula and placula. 

 But in both of these last they are in distinct association and 

 correlate with distinct forms and should be considered as sim- 

 ply exoteric and esoteric cells. They are not true ecto- or 

 endo-blasts until they assume the relations of an external and 

 internal layer as in the gastrula stage. The absence of the pla- 

 cula in many forms may be explained as due to concentration 

 of development. The protected conditions under which the 

 ovum originates makes the constant retention of the placula 

 unnecessary and favors the earlier inheritance of the morula, 

 or mulberry stage ; in fact, any quickening of the processes of 

 growth would bring about this change and the morula stage is 

 only a heaping up of cells into a more massive colonial growth. 

 The rounded globular forms of the morula would thus replace 

 the placula earlier in the life of the embryo and occasion its 

 disappearance in more highly specialized forms, as in the Car- 

 neospongia. 



This theory is apparently very similar to that of Butschli so 

 far as relates to the origin of the placula, but differs in making 

 the morula an important stage of the evolution of forms and 

 in insisting upon the placula as primitively monoplaculate 

 and only secondarily diploplaculate. Butschli's placula is in 

 reality a later stage, a specialized flattened stage of an embryo 

 Metazoon. 



Butschli points out the resemblances of the embryo of Cu- 

 cullanus, Rhabdonema, and Lumbricus to the placula, and the 

 apparently primitive mode of forming the segmentation cavity 

 in the latter by the separation of the two layers is also given 

 in detail by him. Butschli also consider the Trichoplax ad- 

 herens of Schultze, as a living illustration of a full grown, 

 primitive, placulate form. 



We ought to find primitive stages, in the embryos of a prim- 

 itive type and this is eminently the case with Porifera. We 

 should anticipate the opposite with a higher type like the 

 worms or any metameric animal and this appears to be borne 

 out by what Butschli brings forward in support of his theory. 



In Cucullanus the earliest stages are rounded, and we cannot 

 agree with Butschli, that the flattened form which follows this 

 is a primitive placula, or diploplacula. The primitive placula 

 is a single layer which becomes double or diploplaculate and 

 in both stages must precede the morula, and cannot succeed 

 this stage. It will be seen by our remarks above that the 

 esoteric and exoteric differentiations would have occurred nor- 

 mally before the morula stage in the placula of Cucullanus, or 

 else in fusion with it and therefore the double layered placula 

 of Butschli would be necessarily a flattened morula in which 



