Hyatt.] 112 [March 6, 



Archiv. Anat. and Physiol., 1884) ; with regard to this we do 

 not feel able to judge, and Hubrecht's evidence seems to be too 

 incomplete to form a reliable basis for decision on this point. 



The difficulties presented by the posterior openings are appar- 

 ently more serious, but probably these can be satisfied by means 

 of the universal law of concentration in development. The exter- 

 nal opening of the hinder neuropore in the course of these differ- 

 entiations has become more and more narrowed or confined to what 

 must be considered the posterior end of the neural infolding, and 

 more liable to fusion with the blastopore and anus. Thus in some 

 their development may have become really continuous, as 

 claimed by Sedgwick, and in others discontinuous, as claimed 

 by Van Wijhe ; and yet in all cases no discrepancy with the history 

 of the anterior openings in ancestral forms really exists. A 

 parallel case is found in the fusion of the mantle invagination 

 or sac of the Cephalopoda with the primitive shell gland. This 

 takes place in the Sepioidea, according to Lankester, and we have 

 in some measure verified his theory by the aid of Aulacoceras, a 

 fossil form admirably described by Branco (Zeitschr. Deutsch. 

 Geol. Gesellsch., 1880, p. 401) and whose occurrence in a measure 

 was predicted by Lankester. This fusion took place by the ear- 

 lier and earlier inheritance of the loose inclosing mantle sac of 

 the Orthoceratite until it probably began to assume its more mod- 

 ern partly closed form in Aulacoceras of the Trias, leading into 

 the completely closed form it assumed in Belemnites. In the Sepi- 

 oidea it became entirely closed and inherited at so early a stage 

 that the sac first became embryonic and finally fused with the 

 shell gland, thus occasioning the permanent inclosure of the shell 

 in these animals ; whereas in the allied order of Belemnoids the 

 shell probably was external in the young for an appreciable 

 period, and the shell sac originated as a closed fold or invagina- 

 tion later in development and independently of the shell gland. 

 This is also an interesting parallel with the formation of stomo- 

 dea in the Porifera since it is at first an overgrowth or outgrowth 

 of the mantle like that of Aplysia, and other similar cases 

 among Mollusca, which becomes converted into a permanent 

 invagination, thus giving another illustration of the theory here 

 advocated of the origin of ectodermic invaginations as primitive 

 outgrowths which have become transformed into invaginations 



