A. Hyatt — Larval Theory of the Origin of Tissue. 343 



probably no better field for the study of the effects of pressure 

 upon cells than in these cases of transient invagination. It is 

 possible that the invagination stage may be traceable directly 

 to excess of growth in the ciliated cells and their subsequent 

 evaginatiou as outgrowths to the reversal of this process, and 

 at any rate the field is a very promising one in this direction. 



We have also noted in our original essay the probability, 

 that the medullary fold was primitively a stomodeal invagina- 

 tion due to extra growth, and we are able to quote in this con- 

 nection an observation of Dr. Hatschek's in addition to those 

 of Kollmann and Grardiner. 



Dr. Hatschek (Arb. Zool. Inst. Wien, vol. iv, 1881, pp. 45- 

 48) attributes the origin of the primitive segments and other 

 changes of form in the embryro of Amphioxus to the growth 

 and energy of cells. He explains the origin of the medullary 

 plate by differentiations in the cells caused by the extra-growth 

 of the neighboring cells of the ectoderm, and attributes the rise 

 of the ends and final inclosure of the neural canal to lateral 

 outgrowths due to the same cause.* 



The general presence of the different forms of the gastrula, 

 including the planula, indicates, as we have tried to show 

 above, that Haeckel was right in supposing that these stages in- 

 dicated common ancestors for the whole animal kingdom. To 

 this we have also joined the Architroch of Lankester, imagining 

 in common with this author, a very ancient origin for the circles 

 of cilia around the blastopore of the primitive gastrula-like 

 ancestors of the Invertebrata. 



The history of the structural transitions through which the 

 layers of the body pass in their subsequent history sustain the 

 view that the Porifera are the lowest type of Metazoa. The 

 endoderm and ectoderm reach a highly differentiated stage and 

 appear as flat epithelial membranes, but the middle layer re- 

 mains a mesenchyme containing, as stated by all authors, the 

 reproductive bodies of both sexes. The appearance of sperma- 

 tozoa and ova indifferently in the same animal shows, that en- 

 tire separation of the sexes does not take place so far as now 

 known, among Porifera. It is not yet established, that cross 

 fertilization occurs in any form, though there is as yet no 

 grounds for the positive assertion that it does not occur. The 

 history of the early stages exhibits a larval form in which the 

 interior is solid for a certain period and the mesenchyme plays 

 a much more important role than in any other branch of the 

 animal kingdom, as might be anticipated from the adult condi- 



* See also His, "Unsere Korperform," 1875, pp. 60-61, 83 and 178, who has 

 essentially the same idea of the relations of growth of cells and development of 

 organs. 



