584 Bashford Dean Memorial Volume 



However, on still another page of his notebook, Dean lists and briefly describes two 

 ■'Blastulae"' one of which he states measured "44 mm."" in diameter. These were "drawn'" 

 (Figures 4, and 6, plate I), but, since in the original drawings they measure 44 and 48 mm. 

 in diameter, it seems to me that they were surely not blastulae but gastrulae. As such 

 they will be discussed later. 



As has been noted above. Dean went to Japan in 1900 particularly to get material 

 for the embryology of the bull-head shark, Heterodontus. He obtained a large number of 

 its eggs and embryos in various stages of development ^including segmentation). On the 

 egg of this shark. Dean published a short paper (1901.2) entitled "Reminiscences of 

 Holoblastic Cleavage in the Egg of . . . Heterodontus jap07iicus^\ When he unexpectedly 

 began to get embryological material of Chlamydoselachus, the shark assigned by sys' 

 tematists to the lowest position among the Elasmobranchii, Dean not unnaturally looked 

 for similar reminiscences in its eggs. But, neither among his notes nor iinished drawings 

 is there any indication that he obtained eggs in early cleavage stages. 



However, there is some slight evidence that Dean found something that made him 

 suspect the possibility oi holoblastic cleavage in the eggs of Chlamydoselachus. Among 

 his rough pencil sketches I iind a series of four diagrams comparing, in equatorial view, the 

 cleavage patterns in eggs of four different types (Text-figure 22). The first (A) is moder- 

 ately telolecithal but clearly holoblastic. This probably represents a hypothetical 

 ancestral type. The second drawing (B) is labelled '^Chlaynydoselachus'\ It represents 

 an egg vfith a large blastoderm (here defined as a mass of completely formed blastomeres) 

 from which meridional furrows extend without interruption to an imaginary line drawn 

 parallel to the equator and about 35'^ above it. If this line represents the margin of the 

 crerminal area, as it appears to do, then the size of this area is considerably exaggerated. 

 Some of the meridional lines continue further, but are more or less broken. A few reach 

 nearly to the vegetal pole. The third drawing (C) is labelled "Sharks" — evidently 

 meaning typical sharks. It represents an egg with a very small blastoderm and no radial 

 cleavage furrows extending beyond the margin of the mass ot completely formed blasto- 

 meres. The fourth drawing (D), Hke the first, is not labelled. It portrays an egg with 

 a small blastoderm from which many meridional furrows extend to the equator and some 

 beyond it. Those that extend into the lower hemisphere are represented by broken lines. 

 A few of these broken lines reach nearly to the vegetal pole. This drawing evidently 

 represents a type of cleavage reverting from the meroblastic to the holoblastic condition. 

 The cleavage pattern of Chlamydoselachus, as portrayed in the second drawing, bears 

 some resemblance to that of Cestracion as figured by Dean (1901.2). 



But are the lines crossing the margin of the germinal area really cleavage furrows? 

 Among Dean"s records I find three pencil drawings representing in greater detail the 

 circular grooves shov.-n in eggs C. B, and A (Figures 4, 5, and 6, plate I). These pencil 

 drawings show very numerous fine lines crossing the groove in a radial direction. Because 

 of the delicacy of these lines, these drawings are not suitable for- reproduction. I have 

 found similar lines at the margins of the germinal area in the nearly mature ovarian eggs. 



