No. 2.] THE ORIGIN OF THE SEX-CELLS. PA | 
the small nuclei much further from each other, until finally it 
breaks down entirely, and the sexual cells escape. On the 
other hand, the endoderm remains quite as thick, the nuclei 
become smaller, but they are correspondingly more numerous. 
The fact which remains to be decided is whether the bell 
nucleus originates as a whole by delamination, or whether the 
cells migrate from the ectoderm and divide afterwards. 
The cells that pass in from the ectoderm, very soon arrange 
themselves in a double layer. (Fig. 10, c and @.) We here 
see the bell nucleus consisting of the two layers closely applied 
to one another. There seems to be no stage where we find 
either an infolding of the ectoderm, or a solid mass of cells, 
which in later stages form into these two layers. In Fig. 11, 
we see that the double layer of Fig. 10, has become differenti- 
ated into an outer one consisting of a single row of cells, and 
an inner one made up of several rows of cells which stain 
much-more deeply, and are more granular; these are evidently 
the spermatoblasts. In the series Figs. 11, 12, 13, we see the 
direct formation of the spermatozoa. In the first figure the 
spermatoblasts are undergoing division, which is shown by 
the spindle. Karyokinetic figures are of frequent occurrence 
in sections of gonophores at this period of development. This 
fact teaches that by karyokinetic division of these cells the later 
stages are reached. As the series advances, an increase in the 
number of cells and a decrease in the size is noted, until 
finally, we reach, in Fig. 13, a stage where the minute chromatin 
heads with delicate protoplasmic tails are found ; these are the 
mature spermatozoa. Van Beneden in his Pl. I, Figs, 02; 
13, 14, Shows that this spermatogenesis takes place within 
the inner layer of the bell nucleus. He, however, considers 
that this layer is not entirely used up by the formation of these 
sex-cells, but remains as a very thin layer containing very 
minute nuclei, and thus, after the mature spermatozoa are 
developed, the double layer of the bell nucleus may still be 
observed. I cannot deny, or affirm, that such a layer was 
present, for although, at times, I thought I could detect it, 
yet more frequently there was no evidence whatever of its 
existence. 
