S12 NIKOLAS KLEINENBERG. 



time two other small cells separate themselves from the first 

 blastomeres ; the egg consists of six segments, two large and 

 four small. A short time later two more cells are added to 

 the last, and in this manner a little plate of sm^dl Jlattened 

 cells is formed, which, resting upon the top of the two large 

 blastomeres, covers like a roof the gradually widening furrow 

 •which separates them (PI. IX, fig. 1.) 



Such a stage of segmentation is much like one 

 already described in the development of Nephelis, with the 

 difference, however, that this, 'instead of two, possesses 

 three large cells, covered in part by a layer of smaller cells. 

 But there is a still more important difference : while in 

 Nephelis the large blastomeres remain for a long time un- 

 altered, those of humhrlcus soon divide repeatedly, and 

 become blended with the general embryonic mass. At first 

 they separate from one another, leaving in the middle a wide 

 and deep space, one side of which is closed by the curved 

 plate of small cells, whilst the other presents a somewhat 

 restricted aperture opening into the cavity of the capsule. 

 Now the two large blastomeres each divide contempora- 

 neously into two, and at the same time some of the small 

 blastomeres tend towards the centre, interposing themselves 

 between the four large ones. After repeated divisions, which 

 influence the large as well as the sm.all blastomeres, the e.^^^^ 

 assumes a very characteristic appearance ; there are in all 

 sixteen blastomeres, and if sometimes there is one more or 

 less it is always one of the small ones. 



Of the six large blastomeres three are grouped round the 

 aperture of the segmentation cavity, which they have closed 

 or reduced to a narrow slit ; above and alternating with 

 these are the three others, but these do not touch each other, 

 being separated by means of three small blastomeres, which 

 are placed like wedges between them. The ring thus formed 

 of three large and three small blastomeres embraces the seg- 

 mentation cavity, and is covered above by a thin roof com- 

 posed of five or six small blastomeres. Unfortunately such 

 clear arrangements are of too short duration in the first days 

 of development of the animal under consideration. Very 

 soon it comes to pass that by the multiplication, as well of 

 the large as of the small blastomeres, the diflferences in size 

 between them presently disappear, and, further, the intro- 

 duction of the small blastomeres between the off'spring of the 

 large ones, contributes to the destruction of the above-men- 

 tioned order. 



For a time some of the first blastomeres remain upon the 

 surface external to the others, but when these also are dis- 



