5S 



In this diagram (3) the zygote Z is, of course, the product 

 of the united egg and sperm E.S. and the line onwards to 

 the primitive germ-cell U.K.Z. marks the germinal track in 

 Weismann's sense. The whole of the products of the zygote, 

 including those to the left of the line, are assumed by 

 Weismann to be cells of the embryo. 



As we have already seen, the germ-cells are in existence 

 prior to the formation of an embryo, the diagram so far is 

 evidently incomplete. The diagram to U.K.Z. only takes us 

 sufficiently far to account for the asexual generation and the 

 primitive germ-cell. In the case of the female skate 512 

 primary germ -cells have to be provided, and these before there 

 is any embryo at all. The primitive germ-cell U.K.Z. is 

 probably one cell of the tenth cleavage. Only five divisions 

 are shown in the diagram. After its origin it passes through 

 nine further divisions— for convenience only seven are shown 

 in the diagram— 2, 4, 8, 16, 32, 64, 128, 256, 512. These last 

 are 512 primary germ -cells, and are potential female skates. 



Thus far Beard's diagram accounts for a larval — asexual 

 form, viz., the cells to the left of the line Z., U.K.Z. and the 

 primary germ-cells. 



We are now in a position to answer- the question : 

 Whence the embryo ? The embryo is the product of one of 

 the primary germ -cells. " The remainder become the ' sexual 

 " products ' of this same embryo. They may be obliged, as in 

 " the skate, to wander into the embryo, or, owing to the mode 

 "of growth and evolution of the embryo, they may become 

 " enclosed by it." If two or more primary germ-cells develop, as 

 occasionally happens, the result is identical twins, triplets, etc. 



The primary germ -cells which do not give rise to embryos 

 or degenerate are for future generations. After a resting 

 phase, during which the embryo develops, they divide and 

 become secondary germ-cells. With this division "the 

 " beginning of the determination of sex for the following 

 " generation is bound up. After a certain, usually limited, 

 " number of divisions these germ-cells become oocytes or 



