DEVELOPMENT OF ELASMOBRANCH FISHES. 105 



tween them a space which constitutes the commencement of the 

 body-cavity (PL x. fig. 1). From the very first this cavity is more 

 or less clearly divided into two distinct parts; one of them in the 

 vertebral portion of the plates of mesoblast, the other in the 

 parietal. The cavity in the parietal part of the plates alone 

 becomes the true body-cavity. It extends uninterruptedly 

 through the anterior parts of the embryo, but does not appear 

 in the caudal region, being there indicated only by the presence 

 of two layers in the mesoblast plates. Though fairly wide 

 below, it narrows dorsally before becoming continuous with the 

 cavity in the vertebral plates. The line of junction of the 

 vertebral and parietal plates is a, little ventral to the dorsal 

 summit of the alimentary canal (PI. X. fig. 5). Owing to the 

 fact that the vertebral plates are split up into a series of seg- 

 ments (protovertebrse), the section of the body-cavity they 

 enclose is necessarily also divided into a series of segments, one 

 for each protovertebra. 



Thus the whole body- cavity consists of a continuous parietal 

 space which communicates by a series of apertures with a 

 number of separate cavities enclosed in the protovertebrse. The 

 cavity in each of the protovertebrse is formed of a narrowed 

 dorsal and a dilated ventral segment, the latter on the level of 

 the dorsal aorta (PL x. fig. 5). Cavities are present in all the 

 vertebral plates with the exception of a few far back in the tail ; 

 and exist in part of the caudal region posterior to that in 

 which a cavity in the parietal plate is present. 



ProtovertebrcB. Each protovertebra^ or vertebral segment 

 of the mesoblast plate forms a flattened rectangular body, 

 ventrally continuous with the parietal plate of mesoblast. During 

 stage G the dorsal edge of the protovertebrse is throughout 

 on about a level with the ventral third of the spinal cord. Each 

 vertebral plate is composed of two layers, a somatic and a 

 splanchnic, and encloses the already-mentioned section of the 

 body cavity. The cells of both layers of the plate are columnar, 

 and each consists of a very large nucleus, invested by a delicate 

 layer of protoplasm. 



1 No attempt has been made to describe iu detail the dilTerent appearances 

 presented by the protovertebra in the various parts of the body, but in each 

 bfcage a protovertebra from the dorsal region is taken as typical. 



