734 DR. EK. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
small spheres at the second cleavage gradually extended downwards, being successively 
constricted oif from the unsegmented mass. The first resting stage (fig. 64) was much 
less marked than in the individual last described, and in some individuals of this type 
the first period of quiescence is not attended by any flattening of the spheres, though 
a considerable pause always follows the formation of the first four or five small 
spheres. 
7. Lastly, I may describe a very peculiar segmentation, shown by figs. 68 to 72. 
The egg when first observed consisted of three large spheres and four much smaller 
ones. One of the latter soon divided, and the egg passed into a slightly marked 
resting stage (fig. 70). At the next cleavage both large and small spheres divided 
(figs. 71 and 72) without apparent regularity, and the inequality still remained 
marked. In later stages the spheres gradually became more uniform in size, the 
embryo developed normally, and on the following day the free-swimming larva could 
not be distinguished from those produced by more usual forms of development. 
Review. 
The egg may divide at the first cleavage into two, four (2), eight, sixteen, or thirty- 
two spheres, which may be equal or unequal in size. In some cases the egg undergoes 
a preliminary change of form some time before cleavage, without, however, dividing, 
and returning afterwards to a spherical form. The cleavage into eight parts may be 
irregular and incomplete, and at the next cleavage sixteen spheres are formed. 
Cleavage may begin at one pole of the egg with the formation of four or five small 
spheres, and (usually) after a quiescent period the remainder of the vitellus breaks up 
at once or progressively into spheres of approximately the same size as those first 
formed, and the egg passes into the sixteen-sphere stage. 
Lastly, the segmentation may be very irregular as well as very unequal, and follows 
no discernible order. 
I have described the various forms of segmentation in what may seem wearisome 
detail, since the existence of so wide a range of variation in segmentation is quite 
unprecedented, so far as recorded observations show. In the eggs of many animals the 
course of the segmentation appears to be remarkably constant, and the various cleavages 
follow one another with almost mathematical regularity. So far as I am aware, 
Brooks was the first to point out, in the case of the Oyster, in 1879, that the eggs of 
the same species, or even of the same individual, may normally undergo more than 
one mode of development. He described in the Oyster two forms of segmentation, 
of which one was clearly derived by an abbreviation of the other. Intermediate 
forms were not, however, observed, and the eggs could not be said to exhibit 
variation except in one definite direction. In Renilla the eggs vary in many direc- 
tions, and the different forms of development must be due to varying structural 
arrangements within the ege. 
