742 GORHAM. [Vol. XI. 



The Unsegmented Egg. 



The eggs are ellipsoidal, their major axis measuring about 

 0.36 mm. The nucleus is central, surrounded by a mass of 

 radiating protoplasm which is in turn surrounded by coarsely 

 granular yolk, into which the protoplasmic fibres radiate. The 

 chorion is thin and transparent, and, unlike that of many Crus- 

 tacea, is easily penetrated by killing fluids and stains. 



First Cleavage. 



In the great majority of cases the first cleavage-plane cuts 

 the Q%g at its equator, the spindle of the dividing nucleus lying 

 in the major axis (PI. XXXVII, Fig. i). Cases are more or 

 less frequent, however, in which the direction of the spindle is 

 considerably inclined to the major axis (PI. XXXVII, Fig. 2), 

 and in a few cases the direction of the spindle almost coincides 

 with the minor axis of the Qgg, and the cleavage-plane passes 

 through a meridian (PI. XXXVII, Fig. 3). Several eggs have 

 been observed in which cleavage-planes did not appear until 

 four nuclei were present ; the first and second planes in such 

 cases appeared simultaneously (PI. XXXVII, Fig. 4). 



Second Cleavage. 



Soon after the first cleavage-plane is completely formed the 

 two nuclei again divide. This division takes place in one of- 

 two ways, (i) the spindles lying parallel to each other (PI. 

 XXXVII, Fig. 5), or (2) perpendicular to each other (PI. 

 XXXVII, Fig. 6). In either case their position is at right 

 angles to that of the previous spindle and, though intermediate 

 positions may occur, the great majority take one or the other 

 of these two directions. In about seventy-five per cent of the 

 eggs the spindles are parallel, while in the remainder they are 

 perpendicular, thus causing two radically different methods of 

 arrangement of the later blastomeres. We shall follow the 

 course of segmentation in each of these cases separately. 



