STAGES B TO G. ALIMENTARY CANAL. 303 



taken through the back part of the head, the mesoblast plates 

 have nearly the same form as in the sections immediately 

 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 triangular form 

 than in the sections through the trunk (figs. 3 d and 3 e). 



In the section (fig. 3 U) in front of this the ventral portion of 

 the plate is no longer present, and only that part 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 10, 

 fig. 5, taken from the front part of the head of the embryo 

 represented in PL 8, fig. F). 



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

 been described by Oellacher 1 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. 10, fig. 4) ; and indeed their convexity is so 

 great that the space between them has the appearance 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 protovertebrae are present 

 both in Pristiurus and Torpedo embryos. 



The A limentary 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 

 slit-like aperture, which corresponds to the anus of Rusconi 

 (PI- 7, %. 7). 



1 Zeitschrift f. iviss. Zoologie, 1873. 



