DEVELOPMENT OF ELASMOBRANCH FISHES. 107 



muscle-band just described, begins to proliferate, and produce 

 a number of cells, which at once grow in between the muscles 

 and the notochord. These cells are very easily seen both in 

 transverse and lonoitudinal sections, and form the commencinsf 

 vertebral bodies (PL x. fig. 6, and PL XL fig. 10 and 11 Vr). 



At first the vertebral bodies have the same segmentatioa as 

 the protovertebrae from which they sprang ; that is to say, they 

 form masses of embryonic cells separated from each other by 

 narrow slits, continuous with the slits separating the proto- 

 vertebrse. They have therefore at their first appearance a 

 segmentation completely different from that which they event- 

 ually acquire (PL XL fig. 11). 



After the separation of the vertebral bodies from the proto- 

 vertebrse, the remaining parts of the protovertebrae may be 

 called muscle-plates ; since they become directly converted into 

 the whole voluntary muscular system of the trunk. At the time 

 when the cavity of the muscle-plates has become completely 

 separate from the body-cavity, the muscle-plates themselves 

 are oblong structures, with two walls enclosing the cavity just 

 mentioned, in which the original ventral dilatation is still visible. 

 The outer or somatic wall of the plates retains its previous 

 simple constitution. The splanchnic wall has however a some- 

 what complicated structure. It is composed dorsally and ven- 

 trally of a columnar epithelium, but in its middle portion of 

 the muscle-cells previously spoken of. Between these and the 

 central cavity of the plates the epithelium forming the re- 

 mainder of the layer commences to insert itself; so that be- 

 tween the first-formed muscle and the cavity of the muscle- 

 plate there appears a thin layer of cells, not however continuous 

 throughout. 



At the end of the period K the muscle-plates have extended 

 dorsall}^ two-thirds of the way up the sides of the spinal cord, 

 and ventrally to the level of the segmental duct. Their edges are 

 not straight, but are bent into an angular form, with the apex 

 pointing forwards. Vide PL XL fig. 17 mp. 



Before the end of the period a number of connective-tissue 

 cells make their appearance, and extend upwards from the 

 dorsal summit of the muscle-plates around the top of the 

 spinal cord. These cells are at first rounded, but become 



