724 Bashford Dean Memorial Volume 



soon as the capsule is opened, the tension maintained by the albumen is destroyed and the 

 germinal area probably loses its normal position. In nearly every case it remained near the 

 equator of the egg. The albumen is thick and glairy, transparent save at the extreme upper 

 and lower regions [of the capsule?] . Here it becomes opaque and is attached firmly to the 

 capsule. The albumen shows clearly its origin in tunics: one envelope is especially clear near 

 the egg, forming a whitish membrane, reminding one of the inner layer of the albumen of an 

 amphibian egg (e.g., ?<lecturus). When this is ruptured the contour of the egg is disturbed. 

 When the albumen is in part removed, as when the upper portion of the capsule is cut away 

 with the attached albumen, so that the egg is better exposed, there is a relaxed pressure 

 which results in a flattening of the exposed surface of the egg and, in cases, gives rise to the 

 rupture of the vitelline membrane. In such cases the egg appears with a hernia-like expansion. 

 Under usual conditions an egg may be shifted about within the capsule so that the germinal 

 area can be seen. 



As one sees from an inspection of Figures 1 to 6, plate I, there are furrows, travers' 

 ing the region of the upper pole, which are apparently cleavage furrows. Their pattern 

 suggests that this region may be a primary center of development; but it will soon be 

 superseded by the germinal disc, which is already undergoing segmentation and will 

 presently assume complete control. These subjects are fully discussed in the two 

 following sections. 



REMINISCENCE OF TOTAL CLEAVAGE 



The portion of Dean's manuscript dealing with this topic is in a finished condition as 

 compared with most other parts. It is evidently a revision of his article published in 

 1901. In the figures illustrating that article, the outline of the germinal disc, which is 

 small and very faint in the original drawings, does not appear with sufficient clearness to 

 attract the attention of the observer, expecially since it is not labelled. In most of the 

 drawings, as reproduced in 1901, the outline of the germinal disc cannot be seen at all, 

 even with the aid of a strong reading glass. Therefore I have had some of these drawings 

 reproduced by lithography (Figures 1 to 6, plate I), and have inserted here the correspond- 

 ing portions of Dean's manuscript without change save for the rearrangement of some 

 tabulated matter, the substitution of reference numerals to meet a revised arrangement 

 and enumeration of the figures, and the insertion of some additional references to figures. 



The peculiar interest in the development of Heterodontus is that it still bears witness to 

 an earlier condition of holoblastic cleavage. There can be no doubt that the great size of the 

 egg of recent selachians is a secondary embryological character, and that the early ancestors 

 of sharks produced eggs which, like those of ganoids and lungfishes, were small, poor in yolk 

 and fertilized externally. Indeed we already know that the Palaeozoic Cladoselachians and 

 Acanthodians were not provided with intromittent appendages, and that therefore small 

 eggs and a more or less holoblastic cleavage probably then prevailed within the group of 

 sharks. We have found furthermore [Dean, 1906] that a recent Chimaeroid, Chimaera 

 colliei, undergoes in its early stages a curious process of fragmentation of the egg which can 

 best be explained on the ground that it represents a form of holoblastic cleavage, specialized 

 and retained for a new physiological function. On such evidence, accordingly, there was 



