THE RESULTS OF EMBRYOLOGY. 585 



embryo ; moving by its ectodermal hemisphere, and feeding 

 by its endodermal hemisphere. 



The next advance in organization of such a metazoon 

 would doubtless consist in the more complete extension of 

 the protective layer over the nutritive layer, with due pro- 

 vision for the access of the surrounding medium to the latter. 

 It is obvious that this advance might be effected in either of 

 two ways : the one by emboly, the other by epiboly. In the 

 former, the blastopore would be left as the aperture of com- 

 munication of the endoderm with the exterior; and the result 

 would be the formation of an archceostomatous gastrula, such 

 as that which is supposed by Haeckel to be the primitive form 

 of the metazoon. In the latter, the blastopore would com- 

 pletely close up, and a new aperture or apertures must be 

 formed in the ectoderm to subserve the ingestion of nutri- 

 ment. The resulting organism would be a deuterostomatous 

 gastrula. 



Undoubtedly it seems natural to suppose that the first 

 process preceded the second, in order of evolution ; but the 

 proof that it did so is at present wanting. And, however 

 this may be, the progress of inquiry seems to throw more and 

 more doubt upon many cases of the supposed persistence of 

 the blastopore as the mouth. It is certain that, in the great 

 majority of invertebrated animals, the blastopore either be- 

 comes the anus, or closes up ; and renewed observations are 

 needed to determine the limits within which the archaeostoma- 

 tous condition prevails. 



The blastoccele of the gastrula may be obliterated by the 

 approximation of the epiblast and the hypoblast, or it may 

 persist and constitute the perienteron, or primitive perivis- 

 ceral cavity. 



Those animals which, in their adult condition, most nearly 

 represent simple gastrulas with obliterated blastoccele, are the 

 Physemaria and Hydra, cup-shaped bodies with an oval 

 opening at one end, the walls of which are made up simply 

 of an ectoderm and an endoderm. 1 



In the great majority of the Metazoa, a further advance in 

 complication is effected by the appearance, between the epi- 

 blast and the hypoblast, of cytodes, either isolatedly or in a 

 continuous layer, which constitute the mesoblast, and eventu- 

 ally are converted into mesodermal structures. The origin 



1 I do not think that Kleinenberg's fibres in Hydra strictly represent a 

 mesoderm, though they occupy the position of one. 



