CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 25 



The cells do not necessarily always divide in exactly the way described. 

 For example, a new micromere may be constricted off before the one last 

 iormed has divided ; but this does not indicate any essential deviation from 

 the course above described. This is also the case in the apparently divergent 

 method of cleavage seen in Modiolaria and Ostrea virginiana, as was recognised 

 long ago by LOVEN, and was again pointed out by ZIEGDER. In the two 

 Lamellibranchs just named, during the first stages of cleavage, a very 

 remarkable process takes place, a part of the large sphere rising up from it 

 .like a distinct blastomere, but not, like the true blastomeres, entirely separating 

 from it ; at a later stage, this protuberance is withdrawn into the macromere. 

 On account of this process, which is probably determined by the relative 

 distribution of the protoplasm and the yolk in the egg, the first stages of 

 Modiolaria and Ostrea virginiana differ in appearance from the diagrams given 

 .above ; they may, however, be referred to these, as is evident after the de- 

 generation of the false blastomere. 



KAY LANKESTER long ago described the cleavage of the egg in Pisidium 

 pusillum, a form nearly related to Cyclas, into four spheres of equal size, from 

 each of which a smaller cell became constricted (No. 29). If this is really the 

 case, this method of cleavage would not correspond to that known to occur in 

 Bother Lamellibranchs, but would rather closely resemble the cleavage of the 

 Gastropod egg (p. 108). This condition of the egg of Pisidium is however so 

 peculiar when compared with that of other Lamellibranch eggs that it requires 

 : to be further investigated. 



In describing the stages of cleavage, we have so far dealt only with 

 'their outward form. Although the manner in which this arises in 

 the various eggs is very similar, nevertheless certain differences of 

 internal constitution are very soon evident. In one case, a cavity, 

 the cleavage-cavity, soon appears between the micromeres and the 

 .macromeres. [In Cydas, at the 13-celled stage, STAUFFACHER.] 

 This increases considerably in size, as the division of the cells 

 -continues, and leads to the formation of a blastula, such as is found 

 in Cydas, Pisidium and the Unionidae, the wall of which is not 

 uniform in thickness. In other cases, the cleavage-cavity is not so 

 .large, especially at first (Mytilus)* while in Teredo, as well as in 



examination, to be precisely similar to that described above, but neither 

 the first nor the second cleavage separates the animal from the vegetative 

 cells, as BABL asserted, this separation, according to LILLIE, only occurring 

 at a later period, and thus the entoderm arises both from the micromeres and 

 the macromeres. LILLIE suggests that the unequal cleavage in Unio is due 

 to the fact that the rudiment of the immense shell-gland is to be found in 

 the large cell, and he further accounts for the minute size of the entomeres 

 on the ground that the intestine remains undeveloped until a late stage. ED.] 

 * The observations of BABROIS, made on Mytilus (No. 1) are only known to 

 us from the abstract in the Jahrsberichte, but, taken together with the 

 statements of WILSON (No. 59) are probably to be understood in the way 

 indicated above. 



