DIFFERENTIALS IN FERTILIZATION 309 



between the sperm nucleus and the cytoplasm of the ovum. The second type 

 of abnormal interaction represents, on the whole, much the finer test for the 

 mutual fitness of the interacting cells. Thus the spermatozoon may readily 

 enter the ovum when the distance of the two partners in the spectrum of 

 relationship is not too great, but subsequently, incompatibilities between the 

 cells, and especially between their nuclear constituents, may manifest them- 

 selves, or the spermatozoon may be entirely inactivated or eliminated from 

 the ovum, so that a parthenogenetic development of the stimulated egg takes 

 place. 



But there exists within certain limits, in addition, a proportionality between 

 the difficulty which the spermatozoon experiences in entering the egg and 

 the distance in relationship between these two cells. If the distance is very 

 great, for example, when ovum and spermatozoon belong to different classes, 

 it is necessary to make the surface of the egg more sticky by treating it with 

 alkali, according to the method of Jacques Loeb, or by allowing the egg to 

 become stale in order to effect the entrance of the spermatozoon into the 

 ovum. In certain echinoderms, making a dense suspension of eggs, without 

 first washing them in sea water, seems to improve the results in heterofertili- 

 zation (E. Browne Harvey). It is, then, only after this difficulty has been 

 successfully overcome that the more serious antagonism between the con- 

 stituents of the male and female germ cells becomes manifest. If the distance 

 in relationship between egg and spermatozoon is very great, the paternal 

 chromatin is prevented from orderly interaction with the egg chromatin; 

 instead, it is pushed aside into the cytoplasm of the ovum. This reaction may 

 take place almost at once, or it may occur later, during the process of seg- 

 mentation. The subsequent development is, under these conditions, partheno- 

 genetic. However, even such a development is not normal; it appears as 

 though the mere presence of the strange chromatin in the ovum exerts an 

 injurious effect on the latter. Either the development of the embryo may be 

 merely retarded, or certain abnormalities in differentiation may occur and 

 the resulting organism may therefore be less viable than a normal one. Fur- 

 thermore, such organisms, if they should reach the larval stage, do not usually 

 undergo normal metamorphosis. 



From conditions of marked incompatibility between egg and spermatozoon, 

 we find all degrees of transition, to an almost complete harmony between 

 these cells. If in an intermediate stage there is a mild degree of disharmony, 

 only a part of the paternal chromosomes may be eliminated and in the re- 

 sulting embryo the maternal characteristics may predominate over the pa- 

 ternal. As Baltzer and Tennent have shown, elimination of chromosomes 

 may occur at different stages of development: as early as during the first 

 segmentation or later in the blastula stage. According to Tennent, not only 

 paternal but also maternal chromosomes may be eliminated. On the other 

 hand, if sperm and egg are so far removed from each other as to belong to 

 different classes, the paternal chromatin may in some cases be cast out even 

 before the first segmentation. All kinds of irregularities or monstrosities can 

 be observed under these conditions, and in general they are the more severe, 



