334 



PROCEEDINGS OF THE ACADEMY OF [Ap: 



rule, and by observing the position of this furrow the first and second 

 cleavage planes may be kept distinctly in mind until outwardly visible 

 differential changes in the quadrants present other landmarks for orien- 

 tation. 



Origin of Germ Layers. 

 Segregation of the Ectoblast. 



By the next three divisions in which the four macromeres participate 

 the entire ectoblast arises. 



First Quartet. — The sjjindles which precede the appearance of the 

 first quartet of micromeres lie at first nearly radial, their prox- 

 imal ends being distinctly higher than the distal. As a rule, all 

 four spindles do not show the same stage of karyokinetic activity, 

 though irregularities of this nature are not as yet greatly marked 

 (fig. 9). As division proceeds they turn in a dexiotropic direction and 

 with associated cytoplasmic constrictions four small cells are given 

 off toward the animal pole (PI. XXII, figs. 10, 11). These, the first 

 quartet of micromeres, are in size about one-fourth that of their 

 parent macromeres. As they round out in shape they are pushed 

 farther toward the right, and finally come to he in the furrows to 

 the right of the large cells from which they arose. With the com- 

 pletion of cleavage the whole egg again takes on a decidedly rounded 

 contour, the micromeres changing materially in shape, becoming 

 more flattened on their outer surfaces and sharp-angled below to 

 fit the indentations between the macromeres (fig. 14). 



Second Quartet. — The second quartet arises laeotropically, thus regu- 

 larly alternating in direction of cleavage with the first. The derived 

 micromeres are but slightly smaller than the underlying cells from 

 which they arise and are pushed strongly toward the left as they are 

 given off. By this movement the four cells of the first quartet are 

 also carried somewhat to the left, though the rotation is not great. 

 All the second quartet cells are alike in size, there being no sign of 

 increase in D quadrant, as is the case with many Annelids and some 

 Mollusks; nor is there marked difference in their time of origin, though 

 in future cleavages of the egg irregularities in the time at which divi- 

 sions occur in similar cells of the four quadrants become more and 

 more marked. In cytoplasmic structure these cells appear to differ 

 httle from their parent macromeres, though probably they contain 

 less yolk. Their ultimate position is opposite and beneath the divi- 



