KOFOID. — LAWS OF CLEAVAGE. 197 



type of cleavage, for in Echinocyamus (Theel '92), where the 

 cleavage is of the radial type, we find the meridional cleavages, 

 which divide the quartets into "octets," starting with the fourth 

 cleavage (eight- to sixteen-cell stage) at the vegetative pole, and 

 reaching the apical quartet of the animal pole in the sixty-cell 

 stage. Not only does the quartet having the largest cells divide 

 first, but where there is an inequality in the size of the cells of 

 a quartet, the largest cell or cells may divide first, as is the case 

 in Discoccelis (Lang '84) and Unio (Lillie '93). In view of 

 these facts I wish to raise the question as to the applicability to 

 the individual cells of cleaving eggs of the law first formulated 

 by Balfour (To) and concisely expressed in his " Comparative 

 Embryology " ('80, Vol. I. p. 95), as follows : " Where the yolk 

 spherules are fewest, the active protoplasm is necessarily most 

 concentrated, and we can lay down as a general law that the ve- 

 locity of the segmentation in any part of the ovum is, roughly 

 speaking, proportional to the concentration of the protoplasm 

 there ; and that the size of the segments is inversely propor- 

 tional to the concentration of the protoplasm. Thus the seg- 

 ments produced from that part of an egg where the yolk spherules 

 are most bulky, and where therefore the protoplasm is least con- 

 centrated, are larger than the remaining segments, and their 

 formation proceeds more slowly." 



It is true that, of two eggs otherwise similar, the one with the 

 larger amount of yolk in general cleaves more slowly. For ex- 

 ample, Umbrella, which has a large amount of yolk, requires four 

 days to reach the stage attained by Limax in one day. But of 

 two cells of Limax or Umbrella during the cleavage period, that 

 one is the first to divide which is the larger and presumably has 

 the greater amount of yolk, and which in Balfour's terms has its 

 protoplasm less concentrated. Not only this, but in the cases 

 cited the greater the amount of the yolk the greater is the ten- 

 dency of the division of the yolk-laden cell to precede that of the 

 cell with less yolk. There are, to be sure, many cases where 

 Balfour's law actually applies, as in the frog's egg ; but do not the 

 cases cited above, belonging as they do to numerous and widely 

 distributed classes of animals, form an important exception to 

 the law as he has formulated it ? A paradox is thus presented. 

 Yolk appears to delay cleavage in the cells of the frog's egg, 

 to hasten it in the cells of the snail's egg. Yolk also appears to 

 delay the development of an organism as a whole (cf. Limax and 



