Beard, Heredity and the epicycle of the germ-cells. 407 



done at the reduction and sex-determination, which explains why a 

 child, for example, often bears more likeness to a grandparent than 

 to a parent. 



As to the rest of the diagram, this relates to the determination 

 of sex and to the final phases of oogenesis and spermatogenesis. With 

 the exception of the portions relating to the determination of sex the 

 data concerning oogenesis are taken, as will be recognised, from Bo- 

 veri's well known figures. Of course, the embryo is not supposed to 

 be hermaphrodite; both sexes being included in one diagram merely 

 for purposes of convenience. 



For fuller details concerning the determination of sex the reader 

 may be referred to my recent communication on this subject. In the 

 upper part of the diagram, attached to the 55th primary germ- cell, 

 the probable course of oogenesis in the skate is shown. With the 

 final division of the oogonium into two oocytes o. c. the determination 

 of sex is depicted as happening in the formation of male oocytes and 

 a female ones. These enter the period of growth and then pass on 

 to ripen. Lower down, for comparison, the spermatogenesis of Palu- 

 dina, with its two kinds of spermatozoa, is represented after the state- 

 ments of Meves. 



The portions of the diagram, appended to the 55th and 20th primary 

 germ-cells, can naturally be applied to any of the remaining primary 

 germ-cells, other than that, which goes to form the embryo. 



WhatWeismann has termed the ,,germinal track" nowhere here 

 touches the cells of the embryo. Neither, as we have seen, does it 

 really lie within the asexual generation or phorozoon. It is along a 

 line of unicellular organisms, which pass a portion of their life-cycle 

 between one conjugation and the succeeding one within a sterilised 

 individual, formed by the self-sacrifice of one for the good of the rest. 



As revealed by the diagram, throughout this line of unicellular 

 organisms, which are ever such, until one or other of them gets into 

 the cul-de-sac of embryo-formation, there is a direct morphological 

 continuity of germ-cells. 



This is all Nature demands: this she accomplishes by the aid of 

 unicellular organisms. All the observed phenomena of development; all 

 those of heredity are possible in this way 1 ). Notwithstanding apparent 



1) Were proof wanting of the application of the results of the present 

 research even to the highest animals, it might be found in Hubrecht's re- 

 markable researches into the early development of Tup. aja javanica. 

 (A. A. W. Hubrecht, Die Phylogenese des Amnions und die Bedeutuug des 

 Trophoblastes, Amsterdam, 1895.) Here the first products of the egg-cleavage 

 are a small number of cells, forming a sac, the trophoblast, and containing one 

 central cell, out of which the entire embryo arises. 



As is now well-known, Hubrecht homologises the trophoblast with the 



