86 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



far from bending up at this stage, actually bend down in the 

 opposite direction. 



I am at present quite unable even to form a guess what this 

 peculiar feature of the brain means. It, no doubt, has some 

 meaning in reference to the vertebrate ancestry if we could only 

 discover it. The peculiar spatula-like flattened condition of the 

 head is also (vide loc. ant. cit.) apparently found in the Stur- 

 geons ; it must therefore almost undoubtedly be looked upon as 

 not merely an accidental peculiarity. 



While these changes have been taking place in the head not 

 less important changes have occurred in the remainder of the 

 body. In the first place the two caudal lobes have increased 

 in size, and have become, as it were, pushed in together, leaving 

 a groove between them (fig. 8, / s). They are very conspicuous 

 objects, and, together with the spatula-like head, give the whole 

 embryo an almost comical appearance. The medullary canal 

 has by this time become completely closed in the region of the 

 tail (figs. 8 and 8 b]. 



It is still widely open in the region of the back, and, though 

 more nearly closed again in the neck, is, as I have said, flattened 

 out to nothing in the head. 



The groove 1 between the two caudal lobes must not be con- 

 fused (as may easily be done) with the medullary groove, which 

 by the time the former groove has become conspicuous is a 

 completely closed canal. 



The vertebral plates are not divided (vide fig. 7) into a 

 somatopleuric and splanchnopleuric layer by this stage, except 

 in the region of the head (vide fig. 8 , //), where there is a 

 distinct space between the two layers, which is undoubtedly 

 homologous with the pleuro-peritoneal cavity of the hinder 

 portion of the body. 



It is probably the same cavity which Oellacher (loc. cit.) calls 

 in Osseous fishes the pericardial cavity. In the Dog-fish, at 

 least, it has no connection with the pericardium. Of its subse- 

 quent history I shall say a few words when I come to speak of 

 the later stages. 



1 This groove is the only structure which it seems possible to compare with the 

 so-called "primitive groove" of Birds. It is, however, doubtful whether they are 

 really homologous. . 



