134 TRANSACTIONS AND PKOCEEDINGS OF THK [Sehs. lxvi. 



sexual generation, while tlie other will furnish its sexual 

 products.' 



It will doubtless be urged that on my part also it is 

 an assumption that the cells to the left of the line Z to 

 U.K.Z. give origin to the larva. In a sense this is true ; 

 but the one assumption is prima facie as good as the other, 

 and on the further evidences to be adduced it is a good 

 deal better. 



From the existence of a transient nervous system, a 

 blastoderm, and other evanescent structures, the conclu- 

 sion was long ago arrived at that there was a larva or 

 asexual generation in the life-cycle of the skate. From all 

 the known facts of embryology such a larva cannot arise 

 out of an embryo ; it must precede an embryo. There is 

 no embryo by the time the period P.G.C. is reached, the 

 formation of such commencing here. Therefore, the first 

 products of the cleavage, apart from the line leading to 

 U.K.Z., must be the larva. 



Evidence from another side will be found in, for 

 instance, E. B. Wilson's published researches on the 

 development of Nereis? 



There was some hesitation in the writer's mind as to 

 the possibility of using Wilson's results in support of the 

 view here presented as to the nature and destiny of the 

 first cleavage products. A perusal of the lecture cited 

 below served to remove this. His work of 1892 and his 



1 In Ascaris megaloccphala, it is at least possible that the primitive 

 germ-cell is separated off at the fourth cleavage instead of at the fifth. 

 The latter cleavage would then divide the primitive germ-cell into 

 two primary germ-cells, of which the one would go to form the 

 embryo, and the other would represent the " sexual products." If 

 this 1)6 the correct interpretation of the conditions in Ascaris — a point 

 upon which I do not venture to express an opinion — the subsequent 

 division of the cell, regarded by Boveri and others as the primitive 

 germ-cell, would correspond to the formation of secondary germ-cells 

 in Raja ; that is, the parent cell would be a primary germ-cell. 



Regarding the life-history of such a Nematode as Ascaris megalo- 

 cephala, what is written above concerning the part unknown needs no 

 justification. But if it be imagined possible that here, directly from 

 the fertilised egg, the sexual form as it occurs in the horse can arise, a 

 reference to the account of Maupas' results of investigations into the 

 life-histories of a number of Nematoda will dissolve the illusion. 

 {Vide "Arch. Zool. Exper.," v. 8, pp. 463-624, 11 pi., 1900.) 



2 E. B. Wilson, " The Cell-Lineage oi Nereis," "Jour, of Morph.," vol. 

 vi., pp. 361-480, 1892. " Cell-Lineage and Ancestral Eeminiscence," 

 "Wood's Holl Biol. Lectures," pp. 21-42, 1898 (published 1899). 



