262 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



not very active. When it does occur at this time it takes place by 

 karyokinesis. At this stage the egg fits closely into the cavity of the 

 follicle and fills it completely, but immediately after fertilization it 

 becomes distended or enlarged, as shown in Fig. 3, so that the egg fills 

 only about half of it, and there is an empty space, the cavity of the 

 follicle, 5, which, in Salpa hexagona at least, is distal or opposite the pole 

 where fertilization takes place. 



The cells of the wall of this cavity now begin to multiply rapidly 

 by karyokinesis, and nuclear figures may be found in every section, 

 although they are at first confined to that hemisphere which is empty, 

 while that which holds the egg retains for some time longer its epithelial 

 character. The sharp outlines between the cells disappear in the pro- 

 liferating portion, and the nuclei, which have become enlarged and are 

 about to divide, are pushed inwards towards the inner surface of the 

 follicle, as shown in Plate X, Fig. 3. In this figure the follicle is divisible 

 into two sharply contrasted portions, 10 and 7. The hemisphere on the 

 right, 10, is one cell thick and the outlines are distinct. The other hemi- 

 sphere, 7, which for reasons which will appear later I shall call the 

 somatic layer of the follicle, quickly becomes very much thickened by 

 the elongation of the cells and the rearrangement of the nuclei, which no 

 longer lie at uniform distances from the outer surface. The thickening 

 is the result of crowding due to cell multiplication, but it does not result 

 in the production of more than one stratum of cells. The proliferation 

 of follicle cells is most active around the equatorial belt where the two 

 hemispheres meet at the point marked 8 in Fig. 3 and in the following 

 figures. At this point the cells soon begin to push inwards, as shown in 

 Figs. 5 and 8, and to cover up the blastomeres 9, which have in the mean- 

 time been formed by the segmentation of the egg. These are soon com- 

 pletely covered and separated from the cavity of the follicle, Plate X, 

 Fig. 8, 5, by a visceral layer of follicle cells, Fig. 8, 8. This layer appears 

 to be formed from cells which migrate in from the wall of the follicle, 

 although I found in one specimen, Plate IX, Fig. 11, 6, a clearly marked 

 invagination of the wall. I do not know whether this is constant or not. 

 I found no trace of it in other specimens, and Salensky, who has studied 

 the follicle with great minuteness, says nothing of an invagination. It 

 may not be a normal occurrence, although it is so definite and so well 

 marked in this single specimen, that the failure to discover it in others 

 may possibly be due to a failure to obtain sections of a proper stage in 

 the right plane for showing it. 



