1884.] 151 [Hyatt. 



extra growth of the ectoblast, as has been pointed out by Balfour, 

 and in this and in Lumbricus a true epibolic gastrula is built up 

 by this process which is not more primitive than that which occurs 

 in the Ctenophorae or Tubulariae. The gastrula in other words is 

 formed according to a highly concentrated secondary mode of de- 

 velopment, and not by primitive or simple processes. We should, 

 therefore, even while adopting Butschli's theory, decline to accept 

 his typical examples as true illustrations of the theory, and hold 

 rigidly to the law of succession in the stages of the embi^o for 

 justification of this opinion. 



The central cavity of the blastula stage, the so-called Proto- 

 gaster of Haeckel, connects with the exterior by a blastulapore, 

 the " Protostoma" of Haeckel, which is normally closed later in 

 the growth, but remains open for long periods in some sponges, 

 as may be observed in the figures of Sycandra raphanus and in 

 the larva of silicious sponges, as in the embryos of Halichondria 

 and Tethya described above. The assumption, that such a prim- 

 itive cavity necessarily originated as a gastric cavity, seems im- 

 probable. The prototype of this cavity, the aula, must have first 

 appeared as the central hollow in a moving colonial form of Pro- 

 tozoa, simply as a mechanical necessity of the habits and mode of 

 growth, and might have been useful as a float, but was probably not 

 a gastric cavity, but on the contrary similar in every way to the 

 internal cavity of the Volvox-blastula. The additional advantage 

 of the possession of such a hollow in enabling the cells to use two 

 sides instead of one, and to perform the functions of respiration, 

 ingestion and excretion more completely, is obvious. The grow- 

 ing of the cells of the ovum into a hollow sphere, the blastula with 

 its blastulapore. opening externally, is described by Butschli as es- 

 sentially similar to the growth of the adult, floating, spherical colo- 

 nies of Volvox and Eudorina from a single zoon by fission. This 

 author (Bronn's Thierreichs, Protozoa, pi. 45) gives a series of 

 figures illustrating the development of the asexual zoons of Vol- 

 vox, which fully substantiate his conclusions and together with 

 Carter's (cited above), show that the closest comparisons may 

 be made between the early stages of the ovum and those of all 

 forms of Volvox, which is an open blastula like that of some Pori- 

 fera before it leaves the parent colony and becomes free. 



A friend has been kind enough to criticise this conclusion, and 



