DEVELOPMENT OF ELASMOBRAXCH FISHES. 87 



plates have nearly the same form as in the sections im- 

 mediately behind. The ventral continuation of the mesoblast 

 formed by the lateral plate has, however, become much thinner, 

 and the dorsal or vertebral portion has acquired a more tri- 

 angular form than in the sections through the trunk (fig. 3cZ 

 and Se). 



In the section (fig. Sh) in front of this the ventral portion of 

 the plate is no longer present, and only that i3art exists which 

 corresponds with the vertebral division of the primitive plate 

 of mesoblast. 



In this a distinct cavity, forming part of the body cavity, 

 has appeared. 



In a still anterior section, fig. 3 a, no cavity is any longer 

 present in the mesoblast; whilst in sections taken from the 

 foremost part of the head no mesoblast is to be seen (vide 

 PL IX. fig. 5, taken from the front part of the head of the 

 embryo represented in PI. VI. fig. f). 



A continuation of the body-cavity into the head has already 

 been described by Oellacher^ for the Trout : but he believes 

 that the cavity in this part is solely related to the formation 

 of the pericardial space. 



The condition of the mesoblast undergoes no important 

 change till the end of the period treated of in this chapter. 

 The masses of mesoblast which form the tail-swellings become 

 more conspicuous (PI. ix. fig. 4) ; and indeed their convexity 

 is so great that the space between them has the appear- 

 ance of a median groove, even after the closure of the neural 

 canal in the caudal region. 



In embryos of stage G, which may be considered to belong 

 to the close of this period, eighteen protovertebrse are present 

 both in Pristiurus and Torj^edo embryos. 



The Alimentary Canal. 



The alimentary canal at the commencement of this period 

 (stage B) forms a space between the embryo and the yolk, 

 ending blindly in front, but opening posteriorly by a widish 



1 Zeitschrift f. iciss. Zoologie, 1873. 



