OF PLANOCERA ELLIPTICA. 313 



of the mass into two halves, equal or unequal, symmetrical or asymmetrical, though 

 never circular, as represented in figs. 21 and 22. 



In October 1847, Dr. A. A. Gould observed in the eggs of Eolis gymnota, from the 

 Boston harbor, cases in \\'hich the yolk was divided into three spheres. This fact was so 

 new, and I may say, so unexpected, ihatit struck every one then present and interested 

 in embryological researches, as indicating a new law or principle hitherto unnoticed 

 in the division of the yolk. Now, a similar case may be seen here (fig. 29): three 

 perfectly distinct spheres, of unequal size, it is true, but three spheres only. Figs. 27 

 and 28 exhibit the first step of this process. Sometimes, as in fig. 26, when the yolk is 

 divided into two unequal halves, a third smaller sphere may be seen making its np- 

 ]:)earance between them and growing larger, while one of the latter is diminishino-. 



Therefore, the fact that the yolk is sometinies divided into three spheres, during 

 the process of the division, only indicates a modification of the general rule, and not 

 a new rule. 



Next we come to the number four, and instead of four equal parts, we have two 

 small spheres, appearing like hernise in the middle and on both sides of the two halves 

 (fig. 30). Figs. 31 and 32 exhibit two other stages of the same phase. Occasionally 

 the two halves elongate and repeat the process of lateral depressions and grooving 

 (fig. 33). 



Then comes the case with five spheres : a small one, with four of equal size (fif. 

 36). Six and seven will follow (fig. 37), then eight, which coincides with the mathe- 

 matical number. 



I have not seen any case \\\\\\ nine spheres; these, undoubtedly, have escaped my 

 notice. We have ten (fig. 41), and eleven (fig. 42). Beyond these numbers it be- 

 comes difficult, if not impossible, to follow out the irregular process. Even in the 

 regular course of the division, the number of the spheres is difficult to ascertain be- 

 yond sixteen (fig. 43), for, soon appears the mulberry shape (fig. 44), in which the 

 now very small spheres of division are either equal or unequal in size. 



Soon afterwards the mulberry shape itself vanishes, when the eggs reassume a form 

 and an appearance similar to what it was previous to the division, with this difference, 

 however, that the vitelline sphere has become a little larger (fig. 46). 



It has been a matter of some controversy as to whether the spheres of division were 

 surrounded by a membrane or not. When th.e egg was divided into four of these 

 spheres, the latter could be distinctl}^ separated fronj each other (fig. 35). The same- 

 was effectuated when eight spheres were present (figs. 39 and 40). There can be no 

 doubt, therefore, that the spheres of division are perfectly independent of each other. 

 Under a heavy pressure (fig. 40), the peripheric line of the spheres, or else their 

 membrane, seems to penetrate into the mass where the central transparent space.s, 



although not clearly circumscribed by a membrane, are still present. I am inclined 



79 



