372 PROCEEDINGS OF THE ACADEMY OF [April, 
pies the point of greatest anterior extension, while the tip region of 
the anterior arm through which the velum runs lies ventral to the apex 
in the direction of the blastopore (figs. 95, 98). At the same time the 
head end becomes rounded by increased growth of the cephalic area. 
The four original apical cells, as shown in figs. 75 and 76, divide soon 
after and again at a stage represented by fig. 95, so that this region, 
which in Crepidula is in the fully developed veliger still marked by four 
apicals (1a', ete.), here comes to consist of at least twelve very small 
cells, among which no regularity of arrangement is sufficiently marked 
to be of value in orientation. These cells are extremely difficult 
to distinguish from numerous other cells of like form and structure 
which cover the anterior surface of the head vesicle. The apical group 
continues its forward migration in relation to the larva as a whole and, 
as it appears, pushes aside some of the cells which have arisen from 
divisions of the inner and outer basals of the anterior arm, for at a 
later period (fig. 108) the apical group lies close against the first velar 
row. Either such a shifting occurs or the basals become involved in 
the development of the velum. In fig. 108 a row of cells may be 
distinctly observed in which the nuclei are particularly large, extending 
laterally from the apical point. My first thought on seeing them was 
that they were a part of the velum, but after definitely locating the 
position of the apex and following the later history of the velum, it is 
clearly seen that this row never enters into the latter structure, but 
represents in its cell-lineage derivatives of cells of the lateral arms of 
the cross. No ciliation has been discovered in the apical area, and such 
structures are certainly not strongly marked, though without examin- 
ing living material a denial of the possible presence of such structures 
would scarcely be conclusive. 
Nerve and Sense Organs. 
Cerebral Ganglia.—The cerebral ganglia arise at a stage about 
corresponding to fig. 105, though they do not become well marked 
until somewhat later (fig. 108). During this period cells may be 
seen proliferating inward from the ectoderm of the head vesicle 
in the two regions which lie lateral from the apical area. A row 
of cells with large nuclei are at this time plainly visible running 
laterally from the apex, and it is along the anterior side of these 
cells that the ganglia first arise. This row has been identified as 
coming from the lateral arms of the cross, and cells lying between it 
and the anterior portion of the first velar row are from the same source. 
