316 LANGENBECK. [Vou. XIV. 
am convinced that the ventral plate ts formed from the descend- 
ants of the macromeres and over the macromere pole. 
The pole upon which the ventral plate is formed will become, 
as in all other Crustacea, the ventral side of the embryo. The 
cells after reaching the surface rapidly increase in number, 
‘and as the embryo grows backward over the egg it shifts its 
position, so that its long axis finally corresponds with the long 
axis of the egg. In Fig. 29 we can still see that one side of 
the ventral plate is a little nearer the posterior pole than the 
other side, the shifting at this time not having been completed. 
Fig. 35 shows an egg in which, for some reason, the shifting 
has not taken place, so that at this late stage the embryo is 
still oblique upon the egg. In Fig. 33, a stage which is a 
little later than that shown in Fig. 29, the ventral plate has 
grown almost over the posterior pole (Fig. 32), and the axis of 
the embryo lies parallel to the long axis of the egg. 
The rotation of the embryo upon the egg has been described 
by Bergh (93) for Gammarus pulex, in which the embryo lies 
at first parallel to the shorter axis of the egg, and rotates 
during development through an angle of 90°. Bergh said he 
could not explain why it arose in that position. Della Valle 
also figures the embryo as lying at first obliquely over the 
egg, then parallel to the short axis, and, lastly, parallel to the 
long axis. I could not make out, however, whether he sup- 
posed a rotation to take place, or whether he considered the 
short axis to be drawn out at the expense of the long axis. 
In Microdeutopus we have seen that the cause of the oblique 
position of the embryo upon the egg is due to the obliquity of 
the second cleavage plane. 
In the stages represented in Figs. 33 and 34 the outlines of 
the cells are quite sharply marked off from the yolk. Their 
arrangement in definite lines is partly due to the fact that the 
blastodermic cells appear on the surface of the egg arranged 
in definite rows, on account of the regular cleavage of the two 
large macromeres. Yet this cause cannot hold good for the 
sides of the embryo, for in Fig. 28, the side view of an egg 
shown in Fig. 29, the spindles lie at all sorts of angles to each 
other, and the cells have at this time no definite arrangement, 
