CLEAVAGE AND FORMATION OF THE GERM-LAYERS. K'7 



separate the other two (Fig. is A, p. 120) see Conklin, No. IV, pp. 



44-53 on this point]. At the line of junction between the first two 

 blastomeres, the transverse axis of the future embryo can already be 

 seen, while the plane at right angles to this represents the sagittal 

 plane of the embryo. The position of the axes is thus determined 

 very early as was seen to lie the case in other animals.* 



An equatorial furrow cuts off' from these four cleavage-cells four 

 smaller cells so that the embryo now consists of four maeromeres and 

 four micromeres (Fig. 40 C, I-IV and I'-IV"), the latter lying at the 

 animal pole. The relative size of these blastomeres varies con- 

 siderably in different Gastropods. In Patella (Patten, No. 83), 

 and in Paludina, for instance, the micromeres are not much smaller 

 than the maeromeres. whereas in Fulgur, the micromeres in com- 

 parison with the maeromeres, which are very rich in yolk, are hardly 

 visible (McMurrich, No. 70). These differences are no doubt de- 

 termined by the amount of yolk present in the eggs. 



As cleavage proceeds, four more micromeres arise (IJ, ["-IV"), as 

 before, from the maeromeres. As a rule, this process is repeated 

 once more, and in this way three generations of micromeres arise 

 from the maeromeres; this, however, is not the case in all forms. 

 The nuclear spindles in the maeromeres depicted in Fig. 40 A, show 

 that these cells are preparing to divide to form the third generation 

 of micromeres. In the meantime, the already formed micromeres 

 have, by division, increased still further in number ; sometimes, 

 however, they do not multiply until later. In N-ritina, according 

 to BLOCHMANN, the first and second generation of micromeres (I-IV 

 and 1--1-) divide first, but in Planorbis the twelve cleavage-spheres 

 which now compose the embryo undergo almost simultaneous division 

 (Rabl). In the example depicted in Fig. 40 E, the twenty-four- 

 celled stage is reached by a division of the first generation of 

 micromeres quickly followed by the division of the second generation 

 and the abstriction of the third row of micromeres from the maero- 

 meres as indicated by the nuclear spindles in I" -IV" and II \', A. 

 The maeromeres, through this last division, are either considerably 



* [This seems to hold good for all those forms in which the position of the 

 first two cleavages has been investigated in relation to the axis of the adult 

 body. ( onki.in, for instance (No. 25), concludes that, in Crepidula, the first 

 furrow corresponds to the transverse axis and divides the egg into an anterior 

 and a posterior half, while the second furrow lies in the longitudinal axis 

 ami denotes the division of the egg into a right and a left half. Hkymons 

 (No. XII.) finds a similar condition in Umbrella. Cf. also the position of the 

 axis at a somewhat later stage, as given on p. 143. — Kn.J 



