RASHFORD UE AN. 



well marked reminiscence of holoblastic cleavage. And I summarize 

 the present evidence for such a conclusion in the following notes : 



In all early stages of Cestracion examined (I have notes upon up- 

 ward of a hundred examples) the surface of the egg, notably the surface 

 which keeps uppermost, and which we may call the animal pole, is tra- 

 versed by a series of definite and sharply marked lines. These are disposed 

 so conspicuously* that the observer is given the impression that he is 

 examining an egg of Lepidosteus, magnified some 20 diameters. He thus 

 sees at the upper pole of the egg a number of sharply defined areas, 

 resembling the blastomeres in a late cleavage stage of Lepidosteus. He 

 also observes a series of meridional lines passing down the sides of the 

 egg, and diverging from one another radial-wise. These lines are next 

 seen to become groove-like below, and then fade away toward the vegetal 

 pole. There is also similarity in color : the region of the animal pole is 

 of a pale straw color, the vegetal region is greenish yellow, and there is 

 an intermediate equatorial zone, in which there is an orange or pale 

 brownish cast. Thus far it will be seen that the resemblance to the 

 ganoidean egg is a striking one : a more critical examination, however, 

 brings out the facts that in the egg of Cestracion a red-colored germinal 

 disc is present in addition to the above blastomere-hke areas, and that 

 it is situated, not in the centre of the apparent animal pole, but further 

 down on the side of the egg. The homology of the furrows traversing 

 the egg of this shark is accordingly not as self-evident as at first appears. 

 Is it possible to regard them as not due to cleavages, in spite of their 

 extraordinary resemblance ? Or, on the other hand, is the excentric posi- 

 tion of the germinal disc to be interpreted as secondary, due, for example, 

 to some physiological cause ? In attempting to answer these questions 

 I was led to tabulate the reasons favorable and unfavorable for regarding 

 the markings of the egg of Cestracion as due to cleavage characters, and 

 this table may be quoted as follows : — 



* Their conspicuousness may be judged from the fact that they can be seen at 

 a distance of from six to seven feet by persons of normal eyesight. They were dis- 

 tinguished at a distance of nine feet by a sharp sighted laboratory attendant. 



