730 
DR. E. B. WILSON ON THE DEVELOPMENT OE RENILLA. 
differing in no visible respect from those which had developed along the more usual 
course. The matter appeared to me of such interest and importance that I gladly 
availed myself of the aid of two of my fellow workers at the laboratory—Mr. H. L. 
Osborn and Dr. J. Meredith Wilson —in order to study as completely as possible 
the various forms of development. A large number of eggs, produced at different 
times by different individuals, were kept under continuous observation from the time 
of fertilisation up to an advanced stage of the segmentation; they were then proved 
to be capable of full and normal development by isolation in small glass vessels until 
the free-swimming larval stage was attained. We were thus enabled to determine 
with all possible certainty the fact that at least five or six well-marked modes of yolk- 
cleavage, with many minor variations, may occur as normal phenomena of development, 
that the segmentation may be at first equal or unequal, complete or partial, regular or 
irregular, and that a great amount of variation exists in the duration of the various 
stages of activity and quiescence. 
The interval between fertilisation and the first cleavage varies greatly, and is in 
general greater when the temperature is low. Segmentation may begin within ninety 
minutes after fertilisation, or it may be delayed three or four hours beyond this. It 
was found that the longer this preliminary quiescence continued, the more apt the 
eggs were to pass through the less usual modes of development, while those which 
developed promptly were as a rule of the two common types about to be described. 
1. I will first describe a common, though not the most frequent, mode of develop¬ 
ment illustrated by figs. 1 to 18. The egg having remained perfectly spherical from 
the time of fertilisation, becomes of irregular outline, and in two or three minutes 
divides into eight equal segmentation spheres. These are at first imperfectly separated 
(fig. l), but soon become exceedingly distinct (fig. 2). In the individual figured the 
spheres swelled up slightly one minute later (fig. 3), then gradually flattened together 
somewhat, and the egg passed into a slightly-marked quiescent period or “resting 
stage ” (figs. 4, 5). This continued fifteen minutes, when the spheres again swelled 
up, and each divided into equal parts (fig. 6) so that the embryo consisted of sixteen 
spheres. These again flattened together somewhat, and a second resting stage 
ensued (fig. 7) which continued for twenty minutes. The slight swelling of the 
spheres shown in fig. 3 is not accompanied by any visible cleavage. It is probably 
attendant upon some internal change, which may, perhaps, be a division of the nuclei 
—possibly of the spheres also—in a plane parallel with the surface. It is certain 
that such cleavages take place sooner or later, but I have not been able to trace the 
connexion between them and the external signs of activity (see § 2). 
The segmentation now proceeded with great regularity, and appeared from the 
exterior to be regular and complete. Each stage of division, during which the spheres 
are swollen and rounded, was followed by a period of quiescence, in which the spheres 
were flattened and more closely pressed together. This regular alternation of rest and 
