THE MUSEUM. 



55 



whelks within an egg-case was fairlj' 

 constant, about five or six. If the 

 process be mere cannibalism, it is not 

 easy to perceive why it soould stop 

 short, when some live or si.x larvae 

 were left in the egg-case, why, as al- 

 most always happens in the case of the 

 .Alpine StiliiDiandra atra, one should 

 not devour the rest. 



The number is so constant in the 

 whelk, that one may suspect it is gov- 

 erned by some law. Probably too, if 

 one counted them, the number destin- 

 ed to be devoured would be found to 

 be equally constant. The whole pro- 

 cess must take place under the work- 

 ings of a law or laws, and it is part of 

 our tasks as embryologists to try to find 

 out what these are and what deter- 

 mines them. 



If, after the examination of the 

 whelk cases, a fully formed egg purse 

 be taken from the oviduct of a dog- 

 fish or skate, on opening this we shall 

 almost certainly find but a single egg. 



The eggs of different species of dog- 

 fish or skate also differ considerably in 

 size, both as regards the egg-cases and 

 the yolks. These differences are not 

 always in relation to the specific differ- 

 ences in si^e. The smooth skate {R. 

 batis) is much larger than the starry 

 ray. A', radiata) but no proportion 

 can be detected between the sizes of 

 the two skate and those of their eggs. 

 Indeed if we study the ripe eggs of two 

 closely allied species of shark, Mustcl- 

 iis vulgaris and M. Ineiis, we find 

 that, although, the two fishes are al- 

 most of identical size and so alike that 

 no fisherman could ever be e.xpected 

 to distinguish between them, their eggs 

 exhibit great differences in size along 

 with almost inappreciable differences 

 in the texture of the egg-capsule. 



What, then, are the essential differ- 

 ences between the eggs of these two 

 species.' The egg-shell of J/, iiilgiiris 

 appears to be rather thicker than that 

 of .1/. lacvis, and — a very important 

 point — the actual egg, the yolk, of the 

 former is the equivalent in size and 

 weight of about four of the latter. 



This difference cannot be ascribed 

 to the relative sizes of the two species, 

 and, as a matter of fact, it is in asso- 

 ciation with a very fundimental differ- 

 ence in the mode of development. 



Both species arc viviparous, and it 

 is possible that the young are born in 

 both cases in the same condition of 

 development, though this has yet to 

 be determined. In J/, vulgaris the 

 yolk suffices for the whole of the uter- 

 ine development, whereas in M. lacvis 

 it is used up long before uterine life is 

 o\er, and for what is probably a long 

 portion of its uterine existence the 

 young M. laevis is nourished by a sort 

 of placental attachment of its yolk- 

 case to the uterine wall, as recorded 

 by Aristotle, and as rediscovered dur- 

 ing the present century by that great 

 embryologist, Johannes Muller. 



The instances above mentioned may 

 serve as the text for further study. 

 Modern embryology has never yet seen 

 any difficulties or even any problems 

 in the matter at all. We have our 

 classifications of different kinds of eggs. 

 We recognize eggs with little or no 

 food-yolk, and those with much. Var- 

 ious forms of segmentation or cleavage 

 of the egg are distinguished, such as 

 equal or adequal, unequal but com- 

 plete, and meroblastic or partial. Our 

 leading text-books either say, or lead 

 the reader to infer, that the differences 

 between the various forms of cleavage 

 are dependent upon the amount of 

 food-yolk. As compared with the egg 

 of a frog or toad that of a cod-fish con- 

 tains but little food-yolk, yet the for- 

 mer exhibits unequal but complete 

 cleavage, and the latter partial or mer- 

 oblastic. Our classification may be 

 convenient for purpose of elementary 

 instruction, but it is purely empirical 

 and has no real scientific basis. 



It is like everything else. With the 

 advance of knowledge, our conceptions 

 of organic nature become enlarged, 

 and we come more and more to per- 

 ceive how adverse nature is to schem- 

 atic and empirical classifications. But 

 within recent years we have gone fur- 



■ J Ji . W -l M. P 



