140 TItANSACTIONS AND PROCEEDINGS OF THE [Sess. Lxvi. 



embryo has been displaced, in order to establish a more 

 or less problematical larva, and that the germinal track 

 is here somatic. The reply to this is, that the cell U.K.Z. 

 and its immediate ancestors never form part of the larva, 

 and that the period ^ from Z to U.K.Z. — no matter how 

 long it be, whether four generations or four thousand — '■ 

 is marked by a mode of growth and cell-division, con- 

 spicuous by absence in other parts of the diagram." 



This statement requires both elucidation and emphasis. 



The mode of growth of the sphorophyte in plants is 

 essentially apical, that is to say, wherever there is an 

 apex there are always one or more apical cells, which 

 by their division give off products towards the centre. 



In the sexual generation of a Metazoon the mode of 

 growth differs in toto from this ; for here all the products 

 ultimately undergo differentiation, and embryonic or germ- 

 material, corresponding to apical cells, has no existence. 

 The older embryologists of the first half of the nineteenth 

 century thought differently, and some pathologists still 

 cling to their views, but these have no shadow of founda- 

 tion in fact. 



The initial mode of growth and formation of the asexual 

 generation or larva in animals— an organism never of a 

 ' very high degree of organisation — is entirely comparable 

 to that of the sporophyte. As in simple cases of the 

 latter, there is here one " apical cell " which never itself 

 forms part of the larva, but instead thereof gives off into 

 the latter a greater or less number of products, while 

 retaining its own unicellular or Protozoan character. Nor 

 would the conditions be altered if there were several 

 growing points, as generally met with among the 

 Hydrozoa.^ 



1 In the skate this period includes more than five mitoses, probably 

 ten. 



2 Spemann has already compared the mode of origin of the first 

 cleavage products in Nematodes, more especially in Strongylus, to the 

 apical mode of growth in the sporophyte of a plant. He notes that 

 the cell along the line Z-U.K.Z. in my diagram acts as though it 

 were an apical cell of a sporophyte. (H. Spemann, " Die Entwick- 

 lungvon Strongylus paradoxus," "Zool. Jahrb. Marph. Abtheil." vol. 8, 

 p. 304, 1894-95. 



^ It should be mentioned that de Vries and Weismann have already 

 noted the resemblance in mode of growth between the sporophyte and 

 the colonial Hydrozoa. Many of the latter also possess the indefinite 



