324 



The Journal of Heredity 



merely to give a stimulus to the egg-cell 

 and cause the development of the latter ; 

 and (b) that the species-heredity must 

 be carried by the egg, the sperm furnish- 

 ing only Mendelian or relatively su])er- 

 ficial inherited characters, which can 

 be wholly dis])cnscd with. 



GROUND-PLVN OF AN EGG -CELL 



It was formerly thought that the cmljryo 

 was built up out of formless material 

 in the fertilized egg-cell ; and this de- 

 velopment seemed to call for super- 

 natural guidance. But it has now been 

 found that even before the egg-cell is 

 fertilized, it contains the embryo "in 

 the rough:" and the process of develop- 

 ment merely adds the details to the de- 

 sign that is already blocked out. This 

 view makes development seem a much 

 simpler and less mysterious process. 

 The above diagram of a sea-urchin's 

 egg (after Locb) shows within the cell- 

 wall (/) three well-defined zones: (A), 

 a small, clear cap at one pole; (B), a 

 pigmented ring: and (C) another un- 

 pigmented area. It has been found 

 that each of these areas gives rise to a 

 definite part of the embryo, as explained 

 in the text. (Fig. 20.) 



sPEciEs-Hi;Ki';DrrY 



4. This ojjens up an aspect of heredity 

 with which genetics is unable to deal. 

 vStudy which ])roceeds from experi- 

 mental breeding and hybridization, as 

 genetics docs, can only deal with differ- 

 ences in animals or ])lants that can be 

 crossed; it- cannot gra])ple with the 

 greater problem of why one fertilized 

 cell develops into a horse and another 

 that is ai)i)arently not very unlike it 

 develo])S into a lo1)ster or an elm-tree. 

 Genetics has to infer what goes on in the 



cell, by its observation of the characters 

 of the adult organism ; and, as Darbishire 

 remarked, this is like trying to infer 

 the nature of the works of a clock 

 by watching the movement of its hands. 

 Hence the embryologists have, during 

 recent years, been able to make a 

 contribution of fundamental imi)ortance 

 to the study of heredity. 



The most important part of it is the 

 demonstration that the plan or design 

 for the adult organism is laid down in 

 the egg, even before" it is fertilized. 

 Hence there is no longer need to marvel 

 at the intelligence with which the various 

 factors of the egg and sperm cooperate 

 to develop harmoniously into the adult. 

 There is no longer need to imagine any 

 jjurposive or mystical guidance of the 

 ]jrocess of development. The egg starts 

 with a structure outlined in it, and 

 development is merely a ])rocess of 

 growth at \-arying rates, which develops 

 each part of the structure in the jiroper 

 way. 



The existence of this structure in the 

 unfertilized egg was discovered by the 

 late Th. Boveri while studying sea- 

 tirchins. There is some yellowish-red 

 l)igment in the egg, and he noted that 

 this is not equally distributed over the 

 whole stirfacc of the egg, but is arranged 

 in a wide ring from the equator almost 

 to one of the jjoles, as shown in Fig. 20. 

 Thus three zones can be recognized: 

 in the upper one, the skeleton and 

 connective tissue originate; in the mid- 

 dle (pigmented) one, the intestine is 

 built up by sim]jle cell-division; while 

 the third zone gives rise to the outer 

 body-wall. Conklin has found similar 

 structure in the unfertilized eggs of 

 many other low animals. 



FUNCTION OF THE SPEKM 



"The most imj^ortant fact which we 

 gather from these data is that the cyto- 

 plasm of the unfertilized egg may be 

 considered as the embryo in the rough 

 and that the nucleus has ai)i)arently 

 nothing to do with this j)redetermina- 

 tion. This must raise the question 

 .suggested already in the third cha])tcr 

 whether it might not be possil)le that 

 the cytoi)lasm of the eggs is the carrier 

 of the geinis or e\'cn species-heredity, 



