BIGELOW: EAKLY DEVELOPMENT OF LEPAS. 95 



tion of yolk during cleavage has rarely been described. The mass of 

 protoplasm in the yolk-cell after the first cleavage is certainly nearly 

 equal in volume to the next cell (second micromere c*) which will be 

 cut off (see Fig. 27). The same is true for the later cleavages. All 

 these facts, together with those relating to the nucleus which were 

 mentioned in the preceding paragraph, are opposed to the idea of an 

 " emergence of merocytes from the yolk," and support the interpreta- 

 tion which I have given, viz., that all divisions of the yolk-cell are cases 

 of unequal total cleavage. There is nothing to warrant the phrase 

 " emergence of merocytes." 



In concluding this general discussion of the method of cleavage of the 

 yolk-cell, I wish to emphasize the statement that there appears to be 

 no reason for regarding that cell in any of the cleavage stages as essen- 

 tially different in its nature or in its method of division from such well- 

 known examples of yolk-macromeres as are found in gasteropod eggs. 

 So far as I have found, the division of such macromeres is described as 

 differing essentially from that of other cells more rich in protoplasm 

 only in the inequality of the products. Furthermore, I can see no 

 essential difference between the process of cleavage in the yolk-cell of 

 L. anatifera, where there is much yolk, and in that of L. fascicularis, in 

 which there is relatively little yolk, and in which the division is clearly 

 of the ordinary unequal type. 



According to Groom's account ('94, p. 137) a forming or " emerging 

 blastomere " is characterized by a radial arrangement of granules around 

 a clear central space situated near the periphery of the yolk-cell. 

 Groom's Figures 50, 86 and 88 represent this condition. He speaks of 

 the nucleus of the forming blastomere as the centre of the radiation (see 

 his Fig. 14). The clear area seen in a living egg at this stage is certainly 

 not the nucleus, but the astrosphere, tmd the radiations represent an 

 aster. Groom's description of the development of these structures 

 (p. 137) is good. During the division well-marked protoplasmic move- 

 ments give visible evidence of the differential distribution of the cell- 

 substances. The nucleus itself is not easily seen in the living egg at 

 any stage, and certainly is not vesicular at the time when the astro- 

 sphere is clearly defined. Figures 25, 26, and 30 represent sections of 

 eggs in which, when living, the centres of the radiations presented much 

 the appearance shown in Groom's Figures 10-15. The centres of the 

 radiations are seen to be the astrospheres, and the nuclei are repre- 

 sented by the chromatin vesicles, which are certainly invisible in the 

 living egg. 



