288 



SCIENCE. 



[X. S. Vol. XXI. No. 530. 



ihfise stuffs may be analogous to the so- 

 called internal secretions, formed in the 

 ■adult organism by such organs as the thy- 

 iFoid or the sexual glands, which are known 

 %o produce quite specific morphological ef- 

 fects on the body. A second guess is that 

 the formative stuffs may be related to the 

 soluble ferments or enzymes, which in other 

 ways play so great a role in the economy 

 of plants and animals. 



But, aside from this ([uestion, the evi- 

 dence is steadily increasing, I think, that 

 such stuffs exist, that they have a definite 

 .arrangement in the egg, and that in cases 

 where the form of cleavage is constant 

 they are distributed in a definite way to 

 the cells into which the egg splits up. 

 The cleavage-mosaic is accordingly to be 

 conceived as an actv;al mosaic of different 

 materials that are somehow causally con- 

 nected ^Vith the development of particular 

 parts. When these materials are equally 

 distributed by the earlier divisions, as in 

 Amphioxus, each of the resulting cells 

 may upon isolation produce a perfect 

 larva; when they are unequally distributed, 

 as in Dentalium, the cells are no longer 

 ■equivalent, and upon being isolated pro- 

 duce the structures corresponding to the 

 particular stuff's allotted to them.* These 

 facts will presently bring i;s to our first 

 general conclusion. First, if the proto- 

 plasm contain such stuff's, gro\;ped and dis- 

 tributed in a definite way, to just this ex- 

 tent may development receive a mechanical 

 Interpretation— that is, be conceived as the 

 result of an antecedent material configura- 

 tion in the cgg-protophisin. We have as 



* It will appear in tlic sequel tliat even in the 

 latter case the potentiality of pro(lucin<,' a com- 

 plete embryo may still be present in the nucleus. 

 It is important to distinguish between such 

 primary or original nuclear potentiality, wtiich 

 may be common to all the cells, and the secondary 

 or immediate potentiality determined by proto- 

 plasmic spe<'ification. The relation between these 

 is still an unsolved problem. 



yet no very distinct idea regarding the de- 

 gree of complexity of this initial proto- 

 plasmic configuration, though there are 

 facts that indicate that it may not be very 

 great, i. e., that the preloealization is of a 

 somewhat general character. This question 

 appears, however, to be of relatively minor 

 importance in view of an additional con- 

 elusion given by detailed studies on the 

 formation, maturation and early develop- 

 ment of the egg. These studies leave no 

 doubt that the grouping of materials ob- 

 served at the time the egg begins its process 

 of division is not, in some ea,ses at least, a 

 primary or original one, but is of secondary 

 origin. They indicate further that early 

 in the development the egg contains ofily 

 a few of these specific stuffs, at the very 

 beginning possibly none, and that as devel- 

 opment goes forward new stuffs are pro- 

 gressively formed and distributed. Now, if 

 this conclusion is well founded, the actual 

 progressive development of the protoplasm 

 must be conceived as a process of epigen- 

 csis, not of preformation and evolution. 

 This is the first general result that I desire 

 to emphasize ; and it is in harmony with 

 the fact, on which all embryologists have 

 been agreed, since the time of Wolff, that 

 in its obvous features development is by 

 the formation and addition of new parts 

 not previously existent as such in the egg. 

 The embryo is 7iot actually preformed or 

 even predelineated in the protoplasm from 

 the beginning. The protoplasmic stuffs 

 appear to be only the immediate means or 

 efficient causes of differentiation : and we 

 have still to seek its primary determina- 

 tion in causes that lie more deeply. AYe 

 are thus led to a brief consideration of the 

 qiaestion of the physical basis of heredity, 

 which will direct our attention to an ele- 

 ment that has hitherto been disregarded, 

 namely, the mieleus, and bring us to a 

 second general result. 



It was long since suggested by Nageli 



