340 SUMMAKY OF CURRENT RESEARCHES RELATING TO 



mandibular cavities) as far as and including the post-otic somite. The 

 external part becomes, without undergoing structural change, the hypo- 

 blast defining the walls of the pharyngeal portion of the alimentary 

 canal. It is from this hypoblast that the endodermic gill-pouches 

 and their derivatives arise. In ultimate analysis all this arises from the 

 cephalic prolongation, and the formation of this prolongation is there- 

 fore the initial and fundamental process of cephalogenesis. 



The zone of growth plays the essential part in cormogenesis. To its 

 development and differentiation are due the trunk and the tail. As in 

 the head, the ectoblast divides into a medullary plate and an epiblast. 

 The notochord and the mesoblast become defined antero-posteriorly as 

 the embryo elongates. Throughout the whole trunk the hypoblast of 

 the alimentary canal is derived directly from the vitelline endoblast, 

 which arose from the gastrular cleavage. The segmentation of the body 

 begins at the anterior extremity, and proceeds towards the posterior. 

 But after six somites have been thus formed a seventh becomes defined, 

 anterior to the first, in the mesoblast of the head. Behind this cephalic- 

 somite the ganglionic crest of the vagus terminates. 



The author believes that he has succeeded in establishing for Reptiles 

 the three great stages — acrogenesis, cephalogenesis, and cormogenesis — 

 which have already been established for Amphibians, and that this 

 justifies the conception of a general and fundamental law dominating the 

 ontogenesis of all the Craniota. 



Development of Caudal Skeleton in Pleuragramma antarcticum.* 

 A. Knyvett Totton finds that the development of the vertebial column 

 in this Teleostean begins at the caudal end, the hypaxial elements being 

 the first to appear. This coincides with a down-bending of the noto- 

 chord, as in Ichthyosaurians. Epaxial elements do not appear until this 

 condition has given way to the straight condition again. Arches appear 

 as paired cartilages at the sides of the caudal artery and vein, and of the 

 nerve cord. They are separated from the notochord by connective- 

 tissue. 



The notochord is of a relatively enormous size* and persists with 

 only slight constriction throughout life. The centra consists of thin 

 papery lamella? of membrane-bone. Ossification is generally weak. 



The neural and hsernal arches of the penultimate centrum are double, 

 owing to the splitting of single rudiments. Similar phenomena are to 

 be observed in other fishes. They may be produced by splitting of an 

 originally single rudiment (as in Pleuragramma), by crowding of two 

 arches on to one centrum, by fusion of the first epiural apophysis with 

 the last neural arch, by exceptional equal development of both arch and 

 intercalary, or by secondary diplospondyly, i.e. fusion of two centra. 



Large cartilages are present above and below the last two centra in 

 the adult, which support a dorsal and a ventral series of procurrent fin- 

 rays. Their great size is probably connected with the weakness of 

 general ossification. The hypural bones of the adult are formed by a 

 fusion of haemal arches and radials. This compound nature of the 

 hypurals may be seen anteriorly in adult Selachians and in the sturgeon ; 

 lint it has not been shown before in Teleosts. 



* Proc. Zool. Soc, 1914, pp. 251-61 (2 pis.). 



