40 



THE MESOBLAST. 



series of transverse lines of division into mesoblastic somites. Only 

 the dorsal parts of the plates become split in this way, their ventral 

 parts remaining quite intact. As a result of this each plate becomes 

 divided into a dorsal portion adjoining the medullary canal, which is 

 divided into somites, and may be called the vertebral plate, and a 

 ventral portion not so divided, which may be called the lateral plate. 



These two parts are at this stage 

 quite continuous with each other; 

 and the body-cavity originally 

 extends uninterruptedly to the 

 summit of the vertebral plates 



(fig. 21). 



The next change results in 

 the complete separation of the 

 vertebral portion of the plate 

 from the lateral portion ; there- 

 by the upper segmented part of 

 the body-cavity becomes isolated, 

 and separated from the lower 

 and unsegmented part. As a 

 consequence of this change the 

 vertebral plate comes to consist 

 of a series of rectangular bodies, 

 the mesoblastic somites, each 

 composed of two layers, a so- 

 matic and a splanchnic, between 

 which is the cavity originally continuous with the body-cavity 

 (fig. 28, mp). The splanchnic layer of the plates buds off cells to 

 form the rudiments of the vertebral bodies which are at first seg- 

 mented in the same planes as the mesoblastic somites (fig. 22, Vr). 

 The plates themselves remain as the muscle-plates (jtip), and give 

 rise to the whole of the voluntary muscular system of the body. 

 Between the vertebral and lateral plates there is left a connecting 

 isthmus, with a narrow prolongation of the body-cavity (fig. 28 B, st), 

 which gives rise (as described in a special chapter) to the segmental 

 tubes and to other parts of the excretory system. 



In the meantime the lateral plates of the two sides unite 

 ventrally throughout the intestinal and cardiac regions of the body, 

 and the two primitively isolated cavities contained in them coalesce. 

 In the tail however the plates do not unite ventrally till somewhat 

 later, and their contained cavities remain distinct till eventually 

 obliterated. 



At first the pericardial cavity is quite continuous with the body- 

 cavity ; but it eventually becomes separated from the body-cavity by 

 the attachment of the liver to the abdominal wall, and by a horizontal 

 septum in which run the two ductus Cuvieri (fig. 28 A, sv). Two 

 perforations in this septum (fig. 23 A) leave the cavities in permanent 

 communication. 



Fig. 22. Horizontal section through 

 the trunk of an embryo of scyllium con- 

 siderably younger than 28 f. 



The section is taken at the level of the 

 notochord, and shews the separation of the 

 cells to form the vertebral bodies from the 

 muscle-plates. 



c7i. notochord; ep. epiblast; Fr. rudiment 

 of vertebral body; mp. muscle-plate; mp' . 

 portion of muscle-plate already differentiated 

 into longitudinal muscles. 



