1884.] 149 [Hyatt. 



around the ova, and, thus as in the case of internal burrowing 

 parasites, bring about the formation of more or less thick walled 

 cysts. These in time would tend to stiffen and lessen the supply 

 of food, and thus limit the size of the spermatocysts and ova. 

 There are marks of this differentiation in the sponges, some of 

 which have cysts with thick walls composed of several layers of 

 cells as in Aplysilla sulphurea (Von Lendenfeld, Zeit. Wissen. 

 Zool., Vol. xxxviii, p. 262), while the thin walled cysts prevail 

 in others, and probably in all the cysts become single layered 

 and build a more or less stiffened chorion in later stages. Those 

 with thick walls show the transition from the unprotected ovum as 

 it occurred in the higher Protozoa, or possibly even in some 

 sponges, to the inclosed ovum of the higher Metazoa or higher 

 Porifera. The chorion, in other words, is a covering which arises 

 at first as a protection of the tissue against the ova and sperma- 

 tocysts, and then, acquiring through heredity fixed and inherit- 

 able characteristics and thin walls of differentiated cells, becomes 

 finally a highly specialized single layer of epithelium. 



With regard to the meaning of the early stages of the ovum, we 

 come nearer to Biitschli (Morph. Jahrb., 1884), than any other au- 

 thor, and regard his placula theory as opening a way far more prom- 

 ising than any so far proposed. This author, however, voluntarily 

 rejected the aid of the sponges in his arguments, under the erro- 

 neous impression that they were Protozoa. The embryo of the 

 Calcispongiae is, however, as we have tried to show above, a single 

 layered placula or a monoplacula, and is, therefore, directly com- 

 parable with the undifferentiated flat colonies of Protozoa which 

 w r ere more primitive than the blastula form and represent the sim- 

 plest condition of a colony of Protozoa, like Desmarella of Saville 

 Kent. They are, however, devoid of cilia at this stage and there- 

 fore more nearly perhaps represent a mass of amoeboid forms. The 

 formation of the apical or esoteric cells from the cells of the mon- 

 oplacula transforms this stage into a diploplacula, the older or 

 basal cells becoming our exoteric cells. The approximation of 

 the later stage in which true ectoblastic and endoblastic cells first 

 appear to the more primitive and earlier stages in which the 

 exoteric and esoteric cells of the diploplacula are formed can be 

 explained by concentration of development and would necessarily 

 end in the fusion of these two stages, and the ultimate production 



