Habits and Early Dcfclopiiicnt of Liiicrf/cs nicrciiriiis. i6i 



cavity is filled with the gelatinous or fluid substance which forms the 

 ground-substance of the central area of the unsegmented egg (cf. figs. 39, 

 40). Whether this cavity is in any sense an artifact, and if so to what ex- 

 tent, are questions which are difiicult to answer, since the eggs are so opaque 

 in life that their centers can not readily be seen. However, a cleavage cav- 

 ity is a normal feature in the later stages of this and of most other eggs, 

 and since the cavity present along the line of the first cleavage is directly 

 continuous with tliis later cleavage cavity I conclude that it is normal and 

 not an artifact. This cavity is found only in the region of the central area 

 of the egg and it does not extend through the peripheral layers to the sur- 

 face ; furthermore, it does not communicate with the cleavage head, if one 

 may so judge from the fact that the fluid contents of the cavity do not escape. 

 A large part of the ground-substance of the central area of the egg escapes 

 into this cavity during the first cleavage (fig. 41, 43), and most of that 

 which is left in the blastomeres escapes into the cleavage cavity during the 

 second and third cleavages (figs. 44, 45). Owing to the escape of this fluid 

 substance from the blastomeres the latter are left much more compact and 

 with yolk spherules more closely crowded together than in the unsegmented 

 sg& (</ figs. 37, 44). Another evidence that the escape of this central 

 ground-substance into the cleavage cavity is a normal occurrence is found 

 in the fact that although the cleavage cavity becomes quite large, the volume 

 of the entire egg is scarcely greater in the 8-cell or i6-cell stage than in the 

 i-cell stage (cf. figs. 37 and 46). The substance which escapes into the 

 cleavage cavity probably represents a fluid yolk, which is gradually used up 

 in the nourishment of the embryo. 



Second and later cleavages. The subsequent cleavages are fairly regu- 

 lar and in all of them, as far as I have observed, the nuclei divide by mitosis 

 and in a maimer similar to that described for the first cleavage. The divi- 

 sions of the cell bodies are also similar to that of the first cleavage, though 

 some of them merit a special description. 



The second cleavage begins along the line of the first, in the center of 

 the egg, and cuts through to the periphery (figs. 15-18). This cleavage pro- 

 gresses more rapidly on the side of the animal pole than on that of the vegetal 

 pole, with the result that the connecting strand between the daughter cells 

 is left at the periphery of the egg in the vegetal hemisphere (figs. 19, 20) ; 

 later, perhaps by a slight rotation of these cells, this strand is carried still 

 nearer to the vegetal pole (fig. 20). During this cleavage the animal hemi- 

 sphere of the egg is highly arched, the vegetal hemisphere fiat (fig. 19). The 

 4 blastomeres which result from this cleavage are approximately equal in size. 



The third cleavage is equatorial and divides the egg into 8 adequal 

 cells (fig. 21). This cleavage also begins at the center of the egg adjoining 

 the cleavage cavity (figs. 44, 45) and cuts through to the periphery, where 

 a connecting strand is left (fig. 21). This is the latest stage in which I 



