382 Dr. Reid on the Development of the 
of their division; but after this they began to coalesce, though some 
might be separated from the rest still later. Each of the small cells 
into which the ovum had divided measured from the 1-1500th to 
1-2000th of an inch in diameter, and consisted of a cell-mem- 
brane, with several of the minute cells of which the ovum was 
originally chiefly composed placed like nuclei in its interior 
(fig. 12). The ovum at the time of its extrusion may perhaps be 
considered to be a single large cell with a great number of mi- 
nute cells or nuclei in its imterior. During the subdivision of 
this large cell into a number of smaller cells, I could detect no 
changes, after repeated examinations, inthe form of the minute cells 
or nuclei, and none in their arrangement which were not apparently 
produced by the bending inwards of the external or vitelline mem- 
brane to effect the subdivision. I do not mean to assert that 
these central nuclei were not the efficient agents in producing 
these changes ; I only wish to state that I was unable to detect 
any alteration in the form or in the arrangement of these nuclei 
preceding these subdivisions. At the end of the sixth day no 
additional change had taken place in the external form of the 
ovum, but the cells into which it had divided were continuing to 
coalesce, and minute cilia were observed on the upper surface of 
the broad extremity. On the eighth day it had assumed the 
form represented in fig. 13 ; its circumference had become some- 
what translucent, especially at the lower and middle parts, where 
the external layer of cells had separated themselves from the others, 
and coalesced to form the commencement of the shell (fig. 13 a). 
The incipient shell contained many of the minute cells or nuclei, 
and bands of them passed between different parts of its mner 
surface and the dark mass in the interior. The cilia on the broad 
extremity had become larger and more active in their movements, 
and traces were observed of the division of this end into the ci- 
liated discs (fig. 18 c) and the foot (fig. 13 d*). The cells into 
which the ovum had divided had now almost disappeared, while the 
minute cells or nuclei of which the ovum originally chiefly consists 
seemed to be as numerous as ever, and were diffused, except where 
the shell was forming, through a glutinous-looking structureless 
substance. It is now entitled to the term of embryo. Instead 
of describing in their order of succession the different changes 
through which the embryo passes until it leaves the case-mem- 
brane and swims at large, I shall first describe its structure at 
that period, as this will save a good deal of repetition, and also 
render the description more easily followed. Some of the em- 
bryos left their case-membrane about the end of the fourteenth day 
after spawning, but the whole had not escaped until three or four 
* These parts are indicated by the same letters of reference in a more 
advanced stage of their development in fig. 17. 
