226 EMBRYOLOGY 



from the four ventral plates, namely by delamination. Hubrecht derives 

 the epithelium of the young Nemertean from the fusion of these five 

 plates. 



Hubrecht likewise differs from Barrois and Salensky in his descrip- 

 tion of the mode of origin of the proboscis. The latter authors derive it 

 from the secondary ectoderm, but according to Hubrecht it arises from 

 the primary ectoderm and subsequently separates from this and fuses 

 with the secondary ectoderm. When one considers that the lateral 

 organs, according to the concordant statements of Hubrecht and Salen- 

 sky, take their origin from the primary ectoderm, then such a mode of 

 origin of the proboscis might perhaps be intelligible. Small argument, 

 it is true, is to be got from the fact that in the pilidium the proboscis 

 arises from the secondary ectoderm. Pilidium evidently represents the 

 more primitive state, and on this account one must also expect in it the 

 more primitive condition as regards the mode in which the organs origi- 

 nate. As to the lateral plates, which are to be considered as sensory 

 organs, it is easier to suppose that they were already present in the larva, 

 whereas this is scarcely probable for the proboscis. 



The statement of Barrois that the mesenchyma cells are detached from 

 the somatic discs seems to need revision. We have already become ac- 

 quainted with the origin of these cells in the pilidium. In the cephalic part 

 of the embryo they are applied in part to the proboscis (as its musculature), 

 and in part they are arranged in the vicinity of it to form the proboscis- 

 sheath. In this particular also Hubrecht differs from Salensky, for he 

 considers the pocket of the proboscis to be the remains of the blastoccele, 

 whereas Salensky maintains that it arises (as a kind of coelom) by means 

 of a splitting of a mesenchyma-layer. Hubrecht also looks upon the blood 

 lacunae and the cavities of the vessels, the walls of which as well as the 

 body musculature are of a mesenchymatous nature, as remnants of the 

 blastoccele. He likewise derives the fundament of the nervous system 

 from the mesenchyma, a conclusion which is wholly discredited by 

 Salensky, since in the development of Pilidium this observer recognized 

 the nervous system to be ectodermal in nature; this, moreover, agrees 

 with the ordinary mode of origin of this system of organs. On the other 

 hand, Hubrecht is inclined to derive the genital organs, which make 

 their appearance at an early period, from the ectoderm. 



While the proboscis and its sheath have grown considerably 

 farther backv^ards, striking changes have taken place in the 

 intestine. It consists, as in the pilidium, of a posterior 

 wider part and an anterior narrower portion, although in 

 this type even the latter is said to be of entodermal nature. 

 The anterior part becomes solid as the result of cell-growth 

 (Fig. 106), but subsequently is hollowed again, and its lumen 



