346 F. M. BALFOUR. 



auce. The medullary canal has by this thne become com- 

 pletely 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^ between the two caudal lobes must not be 

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

 which by the time the former groove has become con- 

 spicuous is a completely closed canal. 



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

 somatopleuric and splanchnopleuric layer by this stage, ex- 

 cept in the region of the head (vide fig. 8 a, p p'), 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 subsequent history I shall say a few words when I 

 come to speak of the later stages. 



The embryo does not take more than twenty-four hours in 

 passing from this stage, when the head is a flat plate, to the 

 stage when the whole neural canal (including the region of 

 the head) is closed in. The other changes, in addition to the 

 closing in of the neural canal, are therefore somewhat insig- 

 nificant. The folding ofi" of the embryo from the germ has, 

 however, progressed considerably, and a portion of the hind 

 gut is closed in below. This is accomplished, not by a tail- 

 fold, as in Birds, but by two lateral folds, which cause the 

 sides of the body to meet and coalesce below. At the extreme 

 hind end, where the epiblast is continuous with the hypo- 

 blast, the lateral folds turn round, so to speak, and become 

 continuous with the medullary folds, so that when the various 

 folds meet each other an uninterrupted canal is found 

 passing round from the neural into the alimentary canal, and 

 placing these two in communication at the tail end of the 

 body. Since I have already mentioned this, and spoken of 

 its significance, I will not dwell on it further here. 



The cranial flexure commences coincidently with the closing 

 in of the neural canal in region of the brain, and the division 

 into fore, mid, and hind brain becomes visible at the same 

 time as or even before the closing of the canal occurs. The 

 embryo has now become more or less transparent, and proto- 



' 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 horoologoug, 



