Jan. 1902.] BOTANICAL SOCIETY OF EDINBURGH 143 



— the equivalence of all the primary germ-cells, whether 

 their number be 2, 16, 128, 512, or anything else — has 

 been insisted upon. The point is one of the utmost 

 importance, and, therefore, it may be well to once more 

 briefly indicate the grounds for the conclusion. 



All the primary germ-cells have the same ancestry from 

 the primitive germ-cell. One of them forms the embryo ; 

 and there is nothing to show that this one differs in any 

 respect from its sister cells.^ If two primary germ-cells 

 undergo independent development on a blastoderm, the 

 result is, and must be, the production of like-twins. The 

 dermoid cysts or embryomas of "Wilms are, as this able 

 investigator has established, rudimentary embryos. These 

 abnormal embryos must have taken their origin from 

 persistent primary germ-cells ; and the development of an 

 embryoma is embryologically the abnormal formation of a 

 twin, identical with the embryo. 



The likeness of all the primary germ-cells is certain, 

 or almost so ; absolutely nothing suggests unlikeness 

 among them. This essential identity or equivalence 

 of all the primary germ-cells is immensely important 

 from the point of view of heredity. This will be quite 

 obvious. 



It is it, and it alone, which permits of the handing 

 down of the characters of one generation to future genera- 

 tions. It is the very basis of heredity. The formation of 

 like primary germ-cells, and their essential similarity or 

 equivalence, show how, in sexual reproduction, the oflspring 

 resemble their " parents," while differing from them. The 

 likeness in the primary germ-cells leads to likeness in the 

 offspring ; and along with this unlikeness is bound to come 

 in. For the primary germ- cells themselves give rise to 

 secondary germ-cells, which have lost their powers of in- 

 dependent development. It is these, and these only, as a 

 rule, which are present in the finished embryo. They and 

 their progeny are never capable of independent develop- 



1 In Strowjiilus Spemann has commented upon the equivalence of 

 what he terms the primitive germ-cell and the primitive mesoderm- 

 cell ; indeed, he speaks of them as " Geschwisterkind," or cousins 

 (Zool. Jahrb., Morph. Abth." vol. 8, p. 313). His primitive germ-cell 

 is, however, a primary germ-cell ; and the true primitive germ-cell 

 is that from which the' two cells compared together took their birth 



