Febbuaet 24, 1905.] 



SCIENCE. 



287 



whole may be cut to pieces, yet thej-e is no 

 impairment of action, but only a readjust- 

 ment to form smaller systems like the orig- 

 inal whole. The egg, therefore, says the 

 vitalist, can not be an automaton and its 

 development is inexplicable upon a me- 

 chanical theory. 



Such is the paradoxical result to which 

 a superficial comparison of these two cases 

 leads us— a kind of embryologieal anti- 

 nomy, as it were, which at first sight may 

 seem to take away all hope of finding law 

 or order in these phenomena. I will un- 

 dertake to show you speedily that the ap- 

 parent contradiction is easily explicable. 

 I have placed the two cases side by side 

 because each seems to demonstrate the truth 

 of one side of an ancient embryologieal 

 controversy; and we shall presently find 

 reason for the conclusion that each of the 

 opponents, like the two knights and the 

 shield, have recognized but a part of the 

 truth. 



The probable explanation of the difi'er- 

 ence of the behavior between the eggs of 

 Dentalium and of Amphioxus is a very 

 simple one. When we closely study eggs 

 of this type we find that they do not con- 

 sist of homogeneous protoplasm, bvit of 

 different kinds of protoplasmic materials 

 or stuffs that are at the outset arranged, 

 roughly speaking, in horizontal bands or 

 strata, as indicated in the diagram (Fig. 2, 

 B), where the number of strata is arbi- 

 trarily assumed to be four. Now, an ex- 

 amination of the manner in which the egg 

 divides gives strong reason for the conclu- 

 sion that in such forms as Amphioxus the 

 first division bisects these stuffs, so that 

 each of the first two cells receives one half 

 of each stratum (Fig. 2, C, E). In the 

 egg of Dentalium, on the other hand, this 

 is demonstrably not the case, for the lower 

 stratum passes over bodily into one of the 

 cells and is quite excluded from the other 

 (Fig. 2, D). The symmetrical division in 



Amphioxus, the sea-urchin, or the nemer- 

 tine, gives the immediate possibility of pro- 

 ducing two smaller systems similar to each 

 other and to the whole egg. The symmet- 

 rical or qualitative division in Dentalium, 

 on the other hand, does not give such an 

 immediate possibility, for it produces two 

 different systems neither of which is iden- 

 tical with that of the entire egg. It is 

 highly probable that we find here a proxi- 

 mate explanation of the fact that each of 

 the two cells in Amphioxus may produce a 

 perfect dwarf, while in Dentalium neither 

 produces such a larva. Facts like these 

 are leading us to the conclusion that the 

 immediate determining causes of develop- 

 ment are to be sought in specific proto- 

 plasmic stuffs, or organ-forming materials, 

 that are distributed to the cells in a definite 

 way during division. These materials, defi- 

 nitely arranged, are sometimes plainly vis- 

 ible in the undivided egg. I have, for in- 

 stance, been able to show that the egg of 

 Dentalium contains an area of protoplasm 

 at the lower pole that has a causal connec- 

 tion with the formation of the foot and 

 shell, and probably also of the principal 

 part of the mesoblast structures; for if 

 this area be cut off from the unsegmented 

 egg the resulting embryo regularly lacks 

 these structures. In like manner, Pro- 

 fessor Conklin has recently been able to 

 recognize in the protoplasm of the unseg- 

 mented egg of a species of aseidian the 

 material of the future tail-muscles of the 

 larva ; and though no necessary connection 

 between this material and the muscles has 

 thus far been experimentally proved, my 

 experiments on Dentalium leave by analogy 

 little doubt that such a causal connection 

 exists. We do not in the least know how 

 these protoplasmic stuffs or materials act. 

 We can hardly imagine how it is that one 

 kind of stuff involves the development of 

 muscles, others that of nerves, ciliated cells, 

 or shell-secreting cells. We may guess that 



