Larval Theory of the Origin of Tissue. 199 



Protozoa, and holds an essentially distinct idea of what the 

 placula is. The embryo of the Calcispongian is, according to 

 our opinion, a single-layered placula or a monoplacula, and 

 directly comparable with the undifferentiated flat colonies of 

 Protozoa which are more primitive than theblastula form, and 

 represent the simplest condition of an autotemnic colony of 

 Protozoa, like Desmarella of Saville Kent, though not pos- 

 sessed of cilia at this stage, and therefore more nearly perhaps 

 representing a mass of amoeboid forms. 



The formation of the apical or esoteric cells of the upper 

 layer from the cells of the monoplacula transforms this stage 

 into a diploplacula, the older or basal cells becoming our exo- 

 teric cells. True ectoblastic and endoblastic cells first appeared 

 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 

 simply exoteric and esoteric cells. They are not true ecto- 

 or endoblasts until they assume the relations of an external 

 and internal layer, as in the gastrula stage. The absence of 

 the placula in many forms may be explained as due to con- 

 centration of development. The protected conditions under 

 which the ovum originates make the constant retention of the 

 placula unnecessary, and favour 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 Carneospongia. 



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 mono- 

 placulate and only secondarily diploplaculate. Blitschli's 

 placula is in reality a later stage, a specialized flattened stage 

 of an embryo Metazoon. 



Biitschli points out the resemblances of the embryo of 

 CucullanuSy Rhahdonema^ and Luinhricus 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 considers the Tricho- 

 ylax adhcerens of Schultze as a living illustration of a full- 

 grown, primitive, placulate form. 



We ought to find primitive stages in the embryos of a 



