arid Plants . Reducing Division in Metazoa. 455 
lessly at variance with any such idea as that of alternation of 
generations with apospory. It was long ago foreseen that 
here obstacles seemed to block the way. However, when 
looked at in the light of spore-formation, Wilson’s lines of 
cells, mesoblasts, neuroblasts, &c. readily admit of interpre- 
tation, not as due to an actual spore-formation, but as an 
early modification of this, which has already led some distance 
along the path of apospory ; really as a step in advance from 
the former formation of the sexual generation, or gametozooid, 
from a spore-mother-cell, in the direction of its origin from 
a few cells. Carry the process still further, and we obtain 
the counterpart of the primitive streak of the Vertebrata. 
It is very interesting to note that, altogether apart from 
theoretical considerations, Assheton points out how in the 
embryology of the frog and rabbit the first attempts at 
development result in products formed in a totally different 
direction from that subsequently adopted. 
Assheton 1 has really proved that the embryo (i. e. the 
sexual generation) is not formed by the segmentation of the 
egg, but by a proliferation in a totally different direction, i. e. 
in a zone which gradually grows backwards whilst proliferating 
in front. In other words, his researches may be explained 
as showing how the gametozooid arises from an aposporous 
tissue within a larval or asexual generation resulting from 
the segmentation of the egg — from the so-called ‘primitive 
streak.’ 
The pole-mesoderm-cells of Hatschek may also be men- 
tioned, and it may be suggested that a possible interpretation 
of them would be that they might represent spore-mother- 
cells, which had of course undergone no reduction. 
1 Assheton, R., The .Growth in length of the Frog Embryo. Quart. Journ. of 
Microsc. Sci., Vol. xxxvii, N.S. pp. 223-243. 
