Hyatt.] 150 [March 5, 



of a rounded globular and more or less solid form in which it 

 would be difficult to distinguish the exoteric and esoteric cells, or 

 in which this stage of differentiation might be entirely skipped in 

 accordance with the law of concentration in development. The 

 rounded globular forms of the morula would then replace the 

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

 pearance in more highly specialized forms, as in the Carneospongiae. 



This theory is very similar to that of Butschli, 1 so far as relates 

 to the origin of the placula, but differs in making the morula an 

 important stage of the evolution of forms. Butschli points out 

 the resemblances of the embryo of Cucullanus, Rhabdomena, 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 bj^ him. Butschli also con- 

 siders the Trichoplax adhaerens of Schultze, as a living illustra- 

 tion of a full grown, primitive, placulate form. 



We think also, that one ought to find primitive stages in the 

 embryos of a primitive 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, 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 normally be- 

 fore the morula stage in the placula of Cucullanus, or else in 

 fusion with it and therefore the double layered placula of But- 

 schli must be necessarily a flattened morula in which the two 

 layers had already been fused. The relations of the planula 

 stage in Cucullanus and Lumbricus to the gastrula also indicate, 

 that it is simply a modification of the morula stage, and not com- 

 parable with the earlier premorula stages of the embryo. The 

 formation of the gastrula in Cucullanus is a beautiful example of 



1 We first became acquainted with Butschli's views from an article handed us by a 

 friend just before the reading of this paper at the meeting of the American Association, 

 and our ideas with regard to the primitive nature of the sponge placula had been 

 formed and written sometime before this event. 



