592 Bashford Dean Memorial Volume 



advanced stages of embryonic development. It is probable that what Dean meant is that 

 these external gill-filaments are merely temporary modifications of the gill-filaments 

 that persist in the adult; also that they are not so long as they are in other embryonic 

 sharks. This matter of external gills in both embryos and adults of Chlamydoselachus will 

 be taken up fully in a later section of this paper. Hence it need not detain us here. 



In his earlier article on the origin of vertebrate limbs, Osburn (1906) briefly mentions 

 some features in the development of the skeleton of Chlamydoselachus. His later article 

 (1907) on the same subject includes a more detailed consideration of the fin skeletons and 

 pelvis, accompanied by some figures of these structures in a 225-mm. embryo. 



Brohmer ( 190S ) studied the excretory system ot a 25-mm. young embryo ot Chlamydo- 

 selachus. In the stage described, the pronephros is vestigial and the mesonephros is in an 

 early stage of development. 



Ziegler (1908i studied two embryos in the same stage, each 25-mm. long. His 

 paper deals wnth the organogeny, particularly in the head region, with special attention to 

 the "head cavities."" These are cavities which, in elasmobranch embryos, occur in con- 

 nection with mesodermal structures called "head somites," and are regarded as detached 

 portions of the primitive coelomic cavity. For a further discussion see Smith, 1937, pp. 

 391-392. Ziegler w^s unable to find the anterior head cavity discovered by Piatt in 1891 

 in certain other selachians, although he did find an anomalous cavity which he beHeved to 

 be constricted off from the mandibular head cavity. Ziegler described also the infundib- 

 ulum, Rathke's pouch, and the cranial nerves (reconstructed by his pupil, Brohmer). 



Brohmer (1909 • described in more detail the head somites in a 25-mm. embryo of 

 Chlamydoselachus. Like Ziegler, he w^as unable to find the anterior head cavity described 

 by Piatt. Brohmer and Ziegler agree that there is but a single premandibular head 

 cavity in Chlamydoselachus. Brohmer"s contribution, Hke Ziegler's, includes a description 

 of the cranial nerves ot a 25-mm. embryo. 



DESCRIPTIONS OF EMBRYOS FIGURED 



On that page of Dean's notebook labelled "Material & List of Figures" is a Hst of 

 embryos to be drawn. This list has been a partial guide tor this section of the present 

 article — partial only, because not all the embryos there listed were drawn, or if drawn 

 some of the figures have in the long years since been lost. Then again the Hst is only 

 partial because I find in the plates a number of figures not included in the list. Almost 

 every draunng has noted on it the length of the embryo drawn, but some do not. These 

 latter figures are rather ditficult to locate in the series. Again other drawings with. 

 lengths indicated are not on the Hst. In addition to Dean's dra wrings of eggs and embryos, 

 there have been introduced in their proper places, but as text-figures, a few illustrations of 

 embryos described in external aspect by other authors. These fill in gaps in Dean's 

 series and enable me better to show the progressive development of the external form 

 of the embr\'os. 



Owdng to the complete absence of descriptive notes and the almost entire absence of 

 material, the stages of development must be described as they are shown in the individual 



