DISCOPHORA. 291 



History of the germinal layers and development of organs. 



The epiblast. The epiblast is formed of a single layer of cells 

 and early develops a delicate cuticle which is clearly formed quite 

 independently of the egg membrane. It becomes raised into a series 

 of transverse rings which bear no relation to the true somites of the 

 mesoblast. 



The nervous system. The nervous system is probably derived 

 from the epiblast, but its origin still requires further investigation. 

 The ventral cord breaks up into a series of ganglia, which at first 

 correspond exactly with the somites of the mesoblast. Of these, four 

 or perhaps three eventually coalesce to form the sub-cesophageal 

 ganglion, and seven or eight become united in the posterior sucker. 



It would appear from Biitschli's statements that the supra-ceso- 

 phageal ganglion arises, as in Oligochaeta, independently of the ventral 

 cord. 



Mesoblast, It has already been indicated that the mesoblast 

 probably takes its origin both in Nephelis and Clepsine from the two 

 mesoblastic bands which unite in the median ventral line. The 

 further history of these bands is only imperfectly known. They be- 

 come segmented from before backwards. The somites formed by 

 the segmentation gradually grow upwards and meet in the dorsal 

 line. Septa are formed between the somites probably in the same 

 way as in the Ohgocheeta. 



In Clepsine the mesoblastic bands are stated by Kowalevsky to be- 

 come split into somatic and splanchnic layers between which are placed 

 the so-called lateral sinuses. These sinuses form, according to Whitman, 

 a single continuous tube investing the alimentary tract ; a tube which 

 differs therefore to a very small extent from the normal body cavity of 

 the Chsetopocla. The somatic layer of mesoblast no doubt gives rise to 

 the circular and longitudinal muscular layers of the embryo. The former 

 is stated to appear the earliest, while the latter, as in the Oligochseta, 

 first takes its origin on the ventral side. 



A delicate musculature, formed mainly of transverse but also of longi- 

 tudinal fibres, would appear to be developed independently of the meso- 

 blastic bands in Nephelis and Hirudo (Hathke, Leuckart, Robin, and 

 Biitschli). It develops apparently from certain stellate cells which are 

 found between the walls of the alimentary tract and the skin, and which 

 probably correspond to the system of contractile fibres which pass from 

 the body wall to the alimentary tract through the segmentation cavity in 

 the larva of Chsetopoda, various Vermes and Mollusca 1 . 



The mesoblast, so far as is known, gives rise, in addition to the 

 parts already mentioned, to the excretory organs, generative organs, 

 vascular system, etc. 



1 According to Eobin this system of muscles becomes gradually strengthened arid 

 converted into the permanent system. Kathke on the other hand states that it is 

 provisional, and that it is replaced by the muscles developed from the mesoblastic 

 somites. It is possible to suppose that it may really become incorporated in the latter 

 system. 



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