The Embryology of Chlamydoselachus 591 



ology as portrayed in Dean's drawings. This perforce must be a study of the external 

 development of the embryos from the smallest figured (11.5 mm. — Figure 15, plate II) to 

 the largest (390 mm.) — shown in its life colors in Figure 49, plate V. 



It does not lie within the scope of this article, as indicated by its title, to attempt any 

 consideration of the internal development. To be sure, in my account of the blastula and 

 gastrula stages I have included the meager information available concerning their internal 

 structure. But Dean left neither notes nor drawings dealing with the early formation 

 of the embryo and the development of organs. I have found a few serial sections of 

 advanced embryos, but these are in poor condition. Therefore any consideration of the 

 internal development would necessarily be limited to a review of previous contributions. 

 In his article on the anatomy of Chlamydoselachus, Smith (1937) has included references to 

 the scanty literature concerned with the development of organs, and has reviewed certain 

 topics. It will suffice here to indicate briefly, for the convenience of future investigators, 

 the contents of the few publications dealing with the organogeny of Chlamydoselachus. 



RESUME OF RESEARCHES ON THE INTERNAL DEVELOPMENT 



Rose (1895) studied the teeth of a 340'mm. embryo. These teeth were not all in 

 the same stage of development; therefore they afforded a graded series. Rose's obser^ 

 vations indicate that the three large cusps of a typical tooth develop from separate an' 

 lagen; teeth are formed by the union of simple denticles homologous with placoid scales. 

 None of the teeth studied by Rose had attained its final form. 



Nishikawa (1898) figured four transverse sections through the head of a 32'mm. 

 embryo in the region of Rathke's and Seessel's pouches; also a section through the ''grow 

 ing point" of a lateral line of the same embryo. He states that throughout the greater 

 part of the lateral line there is a lumen, which is slit-shaped in transverse sections, but 

 at the posterior extremity it is absent. In the anterior part, where the lateral nerve is in 

 close contact with the anlage of the lateral line, the lumen opens to the exterior at several 

 points. In this connection it should be stated that, as noted by various authors (Smith, 

 1937), in the adult the lateral line is open throughout almost its entire length. 



Dean (1903) published a preliminary report on the embryology of Chlamydoselachus. 

 The date of this paper comes after Dean's first visit to Japan, but before his second visit. 

 Since this article is very brief, and constitutes Dean's only publication on the embryology 

 of Chlamydoselachus, it is here quoted in full: 



In view of the archaic features in the adult, he (Dean] noted as significant in the de- 

 velopment of this form the great depth of the zone of yolk nuclei, the absence of external gills, 

 the more nearly terminal position of the anus, the relatively smaller size of the head, the 

 enormous spiracular cleft and the almost typically fin-fold type of limb. Chlamydoselachus 

 has specialized in the line of producing large eggs, the largest indeed among recent animals, 

 ostrich hardly excepted; that it was, however, until recently an egg-depositing shark is ap- 

 parent from the character of the horn-like capsule (with rudimentary tendriliform processes) 

 which the egg still retains. 



One may query Dean's statement concerning the absence of external gills. In Dean's 

 own drawings, gill-filaments are shown projecting beyond the gill-flaps throughout the 



