STAGES G TO K. THE MESOBLAST. 323 



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 verte- 

 bral and parietal plates is a little ventral to the dorsal summit 

 of the alimentary canal (PL 10, fig. 5). Owing to the fact that 

 the vertebral plates are split up into a series of segments (proto- 

 vertebras), 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 protovertebrae. The cavity 

 in each of the protovertebrae is formed of a narrowed dorsal and 

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

 aorta (PI. n, 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. 



Protovertebrce. Each protovertebra 1 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 protovertebrae 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 proto- 

 plasm. 



Before the end of stage H the inner or splanchnic wall of the 

 protovertebra loses its simple constitution, owing to the middle 

 part of it, opposite the dorsal two-thirds of the notochord, under- 



1 No attempt has been made to describe in detail the different appearances 

 presented by the protovertebrae in the various parts of the body, but in each stage a 

 protovertebra from the dorsal region is taken as typical. 



