294 EMBRYOLOGY 



loosening of the texture of the tissue in successive planes at right angles 

 to the long axis of the body (Fig. 135). As was mentioned, individual 

 cells separate from the muscle-plates, in order to apply themselves (like 

 the splanchnic layer) to the intestine, whereas the remaining part of the 

 muscle-plates supplies the musculature and ei^ithelium of the body-wall. 

 Blood-vessels and segmental organs were not observed in Lopadorhunchus. 

 According to Kleinenbekg's explanation, with which, as regards the ecto- 

 dermal origin of the mesoderm, Salensky also concurs, the primitive 

 mesoderm cells occurring in other Annelids must be looked upon only as 

 early differentiations of ectodermal parts. But when several organs of 

 altogether diffei-ent kinds, such as the musculature, the blood-vascular 

 and excretory systems, can be referred back to such a common funda- 

 ment, then the theory which considers this fundament as a germ-layer 

 is not unwarranted, even when the fundament at times, as in Lopad- 

 urhynchus, makes its appearance in somewhat later stages and in a less 

 primitive manner, namely by the splitting off of a cell-layer from one 

 of the two primary germ-layers. 



Wilson's view should also be mentioned here, according to which, in 

 addition to the two pole cells from which the mesodemial bands arise, 

 three other pairs of similar pole cells are present on the ventral side of 

 the embryos of Liiinhriciig, The three large cells mentioned, from each 

 of which a row of cells extends toward the anterior end of the embryo, 

 lie on either side of the middle line somewhat farther forward than the 

 pole cells of the mesoderm and somewhat more superficial, therefore 

 more in the region of the ectoderm. The imiermost of these three rows 

 is said to constitute the fundament of the nervous system, and the middle 

 one that of the nephridia, whei-eas the significance of the outer one re- 

 mained unknown to the author of this theory.* 



The formation of the mesoderm and body cavity in Enchytr<eoides 

 takes place, according to Roule, in a very singular manner, as far as can 

 be judged from his biief communication. In the "morula," which re- 

 sults from an irregular cleavage, an outer layer, the ectoderm, is split off 

 from a central mass, while the latter separates by means of a similar 

 process into the centrally situated entoderm and the surrounding meso- 

 derm. The former (entoderm) by the appearance of a cavity in it becomes 

 the intestine, while spaces are formed in the mesoderm, which become 

 confluent, and thus give rise to the body cavity between the two layers 



* [The conditions of the mesoderm have been further elucidated by the 

 recent works of Wilson (Appendix to Literature on Annelida, Nos. XXIV. 

 and XXV.) and BfiRon (No. VII.). The origin of the pole cells and their 

 relations to the organs have been followed out further. As a result more 

 definite relations of those parts which were previously held to be exclu- 

 sively mesodermal have been disclosed. A portion of the body muscula- 

 ture appears to be of ectodermal origin. On this point the papers of 

 Beroh and Wilson should be consulted. — K.] 



