58 
one minute and a half the two were entirely separated (Fig- 
ure 60), and in two minutes and fifteen seconds more (Figure 
61) each part was prominent and rounded, and in five min- 
utes more (Figure 62) they had again approached each other,, 
and assumed the form of Figure 13. This is the variation 
which is most frequently met with, and it is plainly a simpli- 
fication of the normal method, by which the result of the 
first period of activity is reached more directly. In another 
variation, which is met with much less frequently, the sec- 
ond period of rest is entirely left out, and the stage shown in 
Figure 18 is reached directly, as shown in Figures 63 to 66. 
In this case the egg passes through the stages 1, 2, 3 and 4, of 
Plate I, but when it assumes the trefoil form of Figure 5, a 
second plane of cleavage, passing through the axis of the polar 
globule, divides each micromere into two, as shown in Figure 
63, in side view, and in 64, viewed from the formation pole. 
Three of these spherules remain distinct, but in a few min- 
utes one of them, which appears to correspond to the second 
micromere of the normal method of segmentation, becomes 
fused with the macromere, as shown in Figure 65, which cor- 
responds to Figure 16 of the normally segmenting egg. A 
few minutes later it assumes the form shown in Figure 66, 
which is clearly the same as Figure 18. Besides these two va- 
riations, which occur quite frequently, and are sufficiently regu- 
lar to demand especial notice, there are occasional irregulari- 
ties, such as are always found, when a number of eggs are 
carefully compared, but these do not call for minute descrip- 
tion. 
THE SIGNIFICANCE OF THE SEGMENTATION OF THE OYSTER EGGS. 
Our information legarding the early stages in the develop- 
ment of Lamellibranchs is very scanty indeed, but so far as 
it goes it indicates that the process of segmentation, as I have 
described it, is, with slight modifications, common to the whole 
class. 
Loven’s memoir (Bidrag till Kannedommcn om Utvecklin- 
gen af Mollusca Acephala Lamellibranchiata. Af S. Lovern 
