1882.] Structure, Development, and Affinities of Plwronis. 375 



mode of origin of the body-cavity as a modification by simplification 

 of the enteroccel type, as described by Kowalewsky, in Argiope.* 



Quite recently Metschnikowf has described the early stages of 

 Phoronis. His account is very different to that given above. In the 

 first place, he has not detected the origin of at least the main part of 

 the mesoblast from the endoderm, as I have described. Further, 

 Metschnikow has figured a blastula with four mesoblast cells in the 

 segmentation cavity. Though I have made numerous complete series 

 of sections through all stages of the blastula, I never have fonnd any 

 cells in the segmentation cavity. I would offer the following expla- 

 nation of Metschnikow's account. 



When the invagination to form the gastrula begins, the hypoblast 

 ceils previously cylindrical become very irregular, and project pseudo- 

 podia-like into the segmentation cavity. The free ends of these cells 

 in actual sections are frequently cut off from their origin, and may 

 then be mistaken for free cells lying in the segmentation cavity. 

 They, however, never contain the nuclei of the cells. I would sug- 

 gest that Metschnikow, who studied the development of Phoronis by 

 means of optical sections of glycerine preparations of the whole 

 larva, has mistaken these projecting ends of the amoeboid endoderm 

 cells for mesoderm cells. 



Rapid growth of the mesoblast " diverticulum " into the prseoral lobe 

 takes place in such a way that distinct somatic and splanchnic layers, 

 applied to the ectoderm and endoderm respectively, are easily to be 

 recognised. 



The cells soon become contractile, and the whole pra?oral lobe 

 almost immediately after its appearance becomes actively so. The 

 muscle cells have all the histological character of Mesenchyme, using 

 this term in the sense used by the brothers Hertwig.| Meanwhile 

 the ectoderm becomes thickened in two regions — 



1, In the prasoral lobe. 



2. In the form of a postoral ring round the mouth. 



The former becomes the future nervous ganglion ; the latter indi- 

 cates the position of the line of future tentacles and the circum- 

 cesophageal nerve-ring of the older animal. 



The anus is formed by a slight invagination of ectoderm behind the 

 postoral ciliated ring on the opposite side of the body to that on 

 which the mouth is placed, and is from the first terminal. The four 

 divisions of the alimentary canal are now apparent, i.e., the hypoblast 

 cells have taken on their characteristic form in the several reoions of 



o 



* A. Kowalewslij, " Protocol of the First Session of the United Sections of 

 Anatomy, Physiology, and Comparative Anatomy at the Meeting of Eussian Natu- 

 ralists in Kasan, 1873 (Russian)." 



f "Z. f. Wiss. Zool.," Ht. IY, 1882. 



X " Die Ccelom-theorie," Jena, 1881. 



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