A. Hyatt — Larval Theory of the Origin of Tissue. 339 



the two layers had already, been formed. 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 comparable with the earlier premorula stages of the 

 embryo. The formation of the gastrula in Cucullanus is a 

 beautiful example of extra growth of the octoblast, as has been 

 pointed out by Balfour, and in this and in Lumbricus a true 

 embolic gastrula is formed by this process which is not more 

 primitive than that which occurs in the Ctenophorse or Tubul- 

 arise. The gastrula in other words is formed according to a 

 highly concentrated secondary mode of development, 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 embryo for justifi- 

 cation of this position. 



We cannot give a better illustration of what we mean by a 

 monoplaculate embryo than Hatschek's Amphioxus in the four- 

 celled stage,* nor of our diploplacula than the same in the 

 eight-celled stage, when the cells of the esoteric layer are first 

 differentiated, which occurs even before the two poles of the 

 embryo become closed and long previous to the stage when the 

 blastula is formed. 



Immediately after the diploplaculate stage the ovum of 

 Porifera and Amphioxus, as well as some other types, presents 

 a stage during which it is a tube open at both ends. The 

 hereditary significance of this stage indicates a tubular ancestral 

 form through which water would freely circulate, and this 

 strengthens our position with regard to the meaning of the 

 aula of the blastula. 



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

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

 the "Protostoma" of Hasckel, 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 raph~ 

 anus, and in the larva of siliceous sponges, as in the embryos 

 of Halichondria and Tethya. The assumption, that such a 

 primitive cavity necessarily originated as a gastric cavity, seems 

 improbable. 



The prototype of this cavity, the aula, must have first ap- 

 peared as a central hollow in a moving colonial form of Proto- 

 zoa, 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 en- 

 * Arbeit, d. Zool. Inst. d. Univ. "Wien, Glaus, iv, bit. I, pi. I. 



