SPECIES ALLIED TO MOTELLA. 25 



The coalescence of two oil-globules has not been directly observed; but 

 eggs having two or more globules have been isolated, and found later with 

 a single globule equal in bulk to the several original globules. The process 

 of coalescence does not require more than a few minutes, perhaps only a 

 few seconds. 



The successive stages in the development of this egg, so far as we have 

 been able to follow it, have each a very characteristic pigment-marking. In 

 the earliest stage of the series (Fig. 1), which is shortly after the closing 

 of the blastopore, we find that the development of black pigment has only 

 fairly begun. The pigment-cells of the yolk-sac are not numerous, and arc 

 mainly confined to the dorsal half, extending to the ventral half only in the 

 region of the tail and near the globule. The pigment is now represented by 

 small, round black dots, each dot marking the centre of a whitish amoeboid 

 cell. Pigment-dots are also distributed along the back of the embryo from 

 end to end, in two irregular lateral lines. 



Eighteen hours later (Fig. 2) the number of pigment-cells is about the 

 same, but the black dots, which were only about .005 mm. in the previous 

 stage, now measure about .02 mm., and nearly fill the cells. They are round, 

 or angular, sometimes following the pseudopodial extensions of the cell. The 

 distribution of pigment on the body is the same as in earlier stages. Scat- 

 tered over the yolk-sac at this time are smaller clear cells, varying in shape, 

 and without a trace of pigment. Fig. 5 represents a similar stage, some- 

 what more highly magnified, found by Mr. Agassiz at Nahant. The size of 

 the globule, and the shape and distribution of pigment, leave no doubt as to 

 its identity with the species found at Newport. Fig. 4 represents a stage 

 intermediate between Figs. 1 and 2, showing two oil-globules slightly un- 

 equal in size. The two together are about equal in volume to the single 

 globule which was found somewhat later in the same egg. 



Forty-two hours after the stage seen in Fig. 1, we find the embryo 

 ready to hatch (Fig. 3). The yolk-sac is much smaller, and the tail nearly 

 reaches the head. The pigment-cells of the yolk-sac have scarcely changed 

 in appearance, but are less numerous. On the other hand, the pigment- 

 cells of the body, with the exception of a single cell on the tail, have 

 assumed a, form quite unlike that seen in the previous stages. They have 

 expanded into large and beautiful dendritic forms. A few of these forms, 

 grayish in color, with delicate processes, are now seen on the surface of 

 the globule. A single large pigment-cell is found in front of each eye, 



