Larval Theory of the Origin of Tissue. 201 



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 Hackel, connects with the exterior by a blastula- 

 pore, the " Protostoma " of Hackel, 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 8ycandra 

 raphamis and in the larva; of siliceous sponges, as in the 

 embryos of Halichondria and Teihya. 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 

 appeared as a central hollow in a moving colonial form of 

 Protozoa, 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 both sides instead of one, and to 

 perform the functions of respiration, ingestion, and excretion 

 more completely, is obvious. The growing of the cells of the 

 ovum into a hollow sphere, the blastula with its blastulapore 

 opening externally, is described by Biitschli as essentially 

 similar to the growth of the adult floating spherical colonies 

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

 author (Bronn's ' Thierreich,' Protozoa, pi. xlv.) gives a series 

 of figures illustrating the development of the asexual zoons of 

 Volvox which fully substantiate his comparisons, and, together 

 with Carter's, 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 Porifera 

 before it leaves the parent colony and becomes free. 



All of these comparisons seem to be much opposed to 

 Biitschli's supposition that the primitive cavity of the blastula 

 originated from a separation of two layers rather than as a 

 stage of development from one primitive layer and the forma- 

 tion of an aula. 



Jn order to account for the differentiation of the esoteric 

 cells we have imagined them as necessarily by position 

 feeding-cells in the ancestors of the di]:)loplaculate stage. In 

 the free morula and closed blastula the same cells or their 

 more modified descendants would tend to retain similar func- 

 tions. The differentiation of the poles would occur in this 

 blastula form according to the same law as is observed in the 

 higher animals, and the tendency already initiated of the zoons 

 of one pole to become exclusively feeding-zoons would be 



Ann. & Mag. N. Hist. Ser. 5. Fo/^, xviii. 14 



