The Embryology of Heterodontus japonicus 725 



strong reason, a priori, for prophesying that in so conservative a shark as Cestracion [Hetero- 

 dontus] there might still — in spite of the great size of the egg — persist traces of the ancestral 

 holoblastism. It was of peculiar interest, therefore, to find such a condition present. 



The earlier stages [Figures 1 to 6, plate I ; and Figure 79, plate VII] invariably showed 

 a series of lines (furrows) traversing the surface of the eggs in a fashion which corresponds 

 closely with the early superficial furrows appearing in the eggs of Amid [Dean, 1906] or, better 

 still, Lepidosteus [Eycleshymer, 1899] — and in these [latter?] instances there is no question 

 that the furrows represent cleavage. 



Before, however, considering the question of the homology of these "cleavage" lines 

 in the egg of Cestracion [Heterodontus] we may describe their conditions in various stages. 

 In the ripe ovarian egg no traces of these lines occur. In an egg taken from the oviduct — the 

 earhest stage in my material — the furrows in the yolk region are already present, almost as 

 numerous as in later stages. But there is this very noteworthy difference, that in the neighbor' 

 hood of the (red) germinal area there appear a number of unpigmented lines and circles 

 [Figures 40 to 43, plate IV] occupying a wide zone between the red germinal area and the 

 yellow yolk region — the most conspicious of these being a white circle immediately sur- 

 rounding the germ [Figures 7 and 8, plate I] . 



As development proceeds, this entire intermediate zone becomes less and less con- 

 spicuous : it is later noted in the early stages of gastrulation. I believe that this zone represents 

 a region in which the interblastomeral spaces of the segmented germ pass over into the furrow 

 spaces on the surface of the yolk region. For this was clearly seen in the earliest stage which 

 I was able to collect (capsule taken from the oviduct); especially clear were the lines when one 

 carefully removes the living germ (e.g. in a spoon-shaped spatula) and examines it (in salt 

 solution) by transmitted light. Such a preparation will be seen in [Figure 7, plate I] from a 

 camera drawing. It shows a stage of late cleavage (? 7-10 cleavage) [more likely sixth to 

 seventh cleavage] with the blastomeres containing the red pigment situated in an irregular 

 central area, and with the surrounding unpigmented band traversed radially by shallow 

 furrows. The latter spread out peripherally and could not be traced further since the soft 

 yolk around the margins had escaped in the preparation. In another and older specimen, 

 from a capsule which was partly protruding from the oviduct, the condition of the marginal 

 zone could be seen more satisfactorily [Figure 8, plate I] . In this, the preparation was partly 

 hardened (sublimate-acetic) before it was examined by transmitted light. There could then 

 be seen not only the central pigmented blastomeres but in the circle surrounding them a series 

 of blastomeres, somewhat larger in outline and separated from one another by wider spaces. 

 Beyond these, and in the region of the yolk, were a number of faintly outlined blascomeres, 

 whose intervening spaces suddenly dilated into the beginnings of the great furrows which 

 traverse widely on the surface of the egg. It may be remarked that stages of or near this 

 period give but the faintest indication of blastomeres in the "transitional zone" if examined 

 as opaque objects, whether in living or in hardened material [Figure 9, plate I] ; and it is clear 

 that the indication of these blastomeres is marked out not by actual separation of the cells, but 

 by shallow superficial grooves and by a thinning away of the cytoplasm in planes in which 

 (perhaps in earlier stages of ontogeny) cell boundaries probably existed. By this process the 

 yolk had become drawn into the central portion of each potential blastomere, leaving the 

 intervening parts transparent — conspicuous when examined by transmitted light. 



Examining now a series of early stages (all drawn from living specimens) we may convince 

 ourselves as to the character and disposition of these larger furrows. In Figures [1 and 2, 

 plate I] are shown two blastulae as they appeared in the open capsule, viewed from above. 

 The germinal area [indicated by a tiny circle] lies nearly or quite at the margin of the 



