152 TKANSACTIONS AND PROCEEDINGS OF THE [Sess, i.xvi. 



high degree of histological differentiation. The sexual 

 generation of plants is at the best a miserable failure from 

 the morphological point of view, and this must be set 

 down to tlie factors already indicated. 



The higher one ascends the smaller it beconjes, until, in 

 the highest flowering plants, it has almost reached the 

 vanishing point, without, however, being able to disappear 

 entirely. 



In animals it is the larva, the phorozoon, or asexual 

 generation, which makes the bravest show in the lower 

 Metazoa ; but even here it is always overshadowed in 

 degree of morphological differentiation by the embryo or 

 sexual generation. In the higher forms it becomes 

 reduced ; but, like the rudimentary sexual generation of 

 the higher plants, it cannot vanish, for it also has its 

 assigned task in the reproductive round. 



The sexual generation, or gametozoon, thanks to the 

 importance of the precious cargo of germ-cells which it 

 carries, has received the kindly attentions of Nature, with 

 consequent higher and higher evolution. From a variety 

 of causes, the larva or phorozoon, on the other hand, tends 

 to simplification the higher one ascends. At the best, its 

 organisation is simple, but even this simplicity leans to 

 meagreness in the Vertebrata as they now exist. 



With the formation of the primary germ-cells the next 

 item in the life-cycle is the production of an embryo or 

 sexual generation by the self-sacrifice of one for the good 

 of the rest. This is indicated in the diagram as having fallen 

 to the lot of the thirty-seventh germ-cell from the bottom. 



In the skate the embryo at first contains no germ-cells, 

 and the primary germ-cells enter it as such ; but — and this 

 is another of the facts established by my work — by the 

 time the embryo is completely laid down, the primary germ- 

 cells divide and form secondary ones ; so that, as a rule, 

 by the time the evolution of the embryo is over and the 

 critical period is reached, the embryo contains only secondary 

 germ-cells, incapable of independent development. 



It may be of interest to record the further fact that in 

 the skate this formation of secondary germ-cells precedes 

 the announcement of the sex of the embryo, and is possibly 

 causally related to it. As we have already seen, the 



