NOTES ON EMBRYOLOGY AND CLASSIFICATION. 413 



incomplete double vesicle tends to bend inward (Fig. 9)^ and 

 to complete the vesicle by closing up (Fig. 10), and it is neces- 

 sary to assume that in the early history of invagination it did 

 quite close. In the primary phase of invaginate development 

 the blastopore was obliterated in due course as cell division 

 went on. The persistence of the blastopore and the establish- 

 ment of a relation sometimes between it and the mouth by 

 means of of the stomodseum^ and sometimes between it and 

 the anus by means of the proctodaeum were later adaptations. 



5. Coincidence of the blastopore with the mouth and with 

 the anus. — On examining the actual developmental histories 

 of Enterozoa which have up to the present time been recorded 

 by means of careful observation we find that by far the ma- 

 jority exhibit the formation of a Diblastula by invagination, 

 the invaginated enteron in many cases consisting of but a 

 few large cells, or even at first of only one large cell. The 

 blastopore closes up in many cases ; it does so in the molluscs 

 Pisidium and Unio, in many Gastropods and Vermes, in 

 Cephalopods, and in Vertebrata. Subsequently, as has been 

 above described for the hypothetical ancestral form, a mouth 

 and an anus eat their way into the completely closed 

 Diblastula by means, respectively, of a stomodseum and 

 of a proctodseum, or of a stomodseal and a proctodseal 

 invagination. 



On the other hand, there are numerous cases in which 

 the blastopore does not close up, but appears to persist as 

 mouth in one set of cases, as anus in another set of cases. 

 Regarding, as I do, the blastopore as an orifice of a secon- 

 dary nature existing solely in relation to the invagination 

 process, and originating after mouth and anus had made 

 their appearance in the progress of animal evolution, I seek 

 to explain its occasional relation to the mouth and to the 

 anus as cases of adaptation. A parallel case of the adapta- 

 tion of an orifice of invagination to functional purposes will 

 be useful for my argument. The primary optic vesicle, like 

 the nerve ganglion-masses, was originally developed by de- 

 lamination in higher animals, but has in many cases taken 

 to a development by invagination. In the Cephalopods the 

 vesicle presents at an early stage a wide rim or margin, 

 which gradually closes in leaving for a time a small orifice 

 comparable to the blastopore of an invaginate diblastula. 

 This orifice is obliterated in the Dibranchiate Cephalopods, 

 but in Nautilus it is seized upon by adaptation and made use 

 of as the chief optical condition of the whole ophthalmic 

 apparatus. It serves in place of a lens or refractive body, to 

 produce an image on the retinal surface in virtue of its pin- 



