Ova of the Nudibranchiate Mollusca. 381 
bilamellata, D. tuberculata and Dendronotus arborescens, one 
ovum is generally contained in each case-membrane ; but in some 
portions of the spawn of the Doris bilamellata, two and even three 
ova were found in the same case-membrane. The greater number 
of the case-membranes in the spawn of the Eolis contained each 
two, three, four, and even five ova. The size of the case-mem- 
branes varies according to the number of ova which it contains. 
In the spawn of the Holis the shape of the case-membranes 1s 
easily altered by external pressure, so that instead of being cir- 
cular or oval they were frequently multangular. 
One and sometimes two small transparent cells were seen in 
some of the ova examined soon after being spawned, adhering 
feebly to the outer surface of the external membrane (fig. 2 a) at 
the line of the first division of the ovum. These transparent cells 
were in general very easily detached by pressure, and were some- 
times seen lying loose within the case-membrane. 
I shall first describe the changes observed im the ova of the 
Doris bilamellata in the course of their development. <A few 
hours after the extrusion of the spawn, a transverse groove pre- 
sents itself on the surface of the ovum (fig. 4), and this gradually 
deepens and divides it into two equal parts, each part being of a 
circular form and completely inclosed in its own external mem- 
brane (fig. 5). Each of these again undergoes a bipartite di- 
vision (fig. 6), and the four equal parts of which the ovum now 
consists proceed in their turn to divide in the same manner 
(fig. 7). In a portion of spawn examined between eleven and 
twelve hours after its extrusion, all the ova, with the exception of 
a very few which presented the primary division into two, had 
divided into four parts ; and eight hours after this, or about nine- 
teen hours from the time of spawning, most of them had divided 
into eight, some still consisted of four, and a few of six parts. 
Examined after twenty-seven hours from the time of spawning 
the ova had the appearance represented in fig. 8, and after fifty 
hours the appearance represented in fig. 9*. At the end of about 
seventy-five hours the cells of the ovum were still more subdi- 
vided, as is shown in fig. 10. Between this period and the end 
of the fifth day, the division of the cells of the ovum appeared to 
have reached its utmost limit, and the ovum gradually changed 
its shape, becoming somewhat elongated and broader at one end 
and narrower at the other, as in fig. 11. Up to this time these 
cells, though adhering by those parts of their external surfaces in 
contact, could sometimes be detached from each other by the ap- 
plication of pressure—more readily however at the earlier periods 
* It is necessary to employ afresh portion of spawn in each observation, 
as the development of the ova in those portions of the spawn used in such 
examinations under the microscope is very seriously disturbed. 
