Hyatt.] 140 [March 6, 



The figure, however, would be round instead of elliptical and 

 there would be no nervous ring above and around the actinostome 

 (stomodeum), which would also be circular. We should also go a 

 step farther and trace under this diagram another in which an 

 embryo in similar section w^ould appear without an actinostome. 

 The mouth would be presented as an open, enlarged blasto- 

 pore leading into a ciliated cavity, the walls of which would 

 be porous and presented as triple, but with extremely thin 

 mesenchyme, and the cells of the ciliated endodermic and 

 smooth ectoderm ic cells almost in contact on the severed 

 parts, and actually in contact in each pore. This diagram would 

 correspond with an ideal horizontal section through an ascon if 

 supposed to have the mouth and blastopore coincident, and if 

 deprived of the skeletal spicules, or to a similar ideal section 

 through a larval sycon in the ascon stage of development when 

 without spicules. This ideal form would be essentially a gastrula 

 perforated with pores, but consisting of three layers and not of 

 two layers as imagined by Haeckel, Huxley and Sedgwick. 



Thus although obliged to differ in important details from these 

 authors our results are approximately similar. In other words 

 the gastrula theory appears to be supported by both the observed 

 gradations of structure and those of embryology among the Porif- 

 era ; and the adult ascon, the lowest Metazoan, is a triplohlastic 

 gastrula differing only from the ideal gastrula in being perforated 

 with pores. 



Recapitulation and Remarks. 



The tissues of Metazoa are, as is now generally recognized, 

 built up by the parthenogenetic fission of cells ; and it is evident 

 that the massiveness of tissue is due to the extremely rapid 

 growth of cells by this mode. They form membranes or masses 

 united in forms similar to Desmarella, and not in forms similar to 

 those colonies of Protozoa which are bound together by means of 

 plasma or by stalks. In so far as this morphological result is 

 concerned, therefore, the cells of the tissues are completely 

 divided, and represent only the most exceptional structural results 

 of agamic fission when compared with the formation of colonies 

 among Protozoa. 



