38 PROCEEDINGS OP THE AMERICAN ACADEMY 



III Tylenchus, AnguiUula, and Ehahditis doliclmra, according to 

 Butsclili," the case is different; for here the first cleavage, although at 

 right angles to the longer axis of the ellipsoid, is meridian, if the exit 

 of" the polar globule determines the position of the axis of the ovum. 



From the foregoing examples it will be seen that the exceptions in 

 the direction of the first cleavage-plane are neither so numerous nor so 

 serious as might at first be supposed. For the most part they are 

 cases in which the data for exact orientation have escaped detection, 

 either because the dilliculties in the way of direct and connected 

 observation were not surmounted, or because the attention of the ob- 

 server was not brought to bear on the point we are here considering. 



But why, it may be asked, should there be any uniformity in the 

 direction of the first cleavage-jjlane ? What are the antecedent con- 

 ditions or relations which determine this uniformity ? It is a well- 

 known fact that the nuclear spindles of one cell-generation tend to 

 arrange themselves at right angles to those of the preceding genera- 

 tion. The primary spindle (archiaraphiaster), at the moment when it 

 divides to form one of the polar globules, almost invariably coincides 

 with the axis of the ovum ; * and hence the first cleavage-?i)indle 

 usually assumes a position perpendicular to this axis, and the corre- 

 sponding cleavage falls in a meridian plane. The primary or " ma- 

 turation spindles" assume a radial position, evidently in obedience to 

 what is known as the polarity of the ovum ; and the arrangement of 

 successive spindles is regulated by the polarity of the blastomeres to 

 which they belong, and by the interaction of these polarities. The 

 uniformity in the direction of the first cleavage plane is tlien only an 

 outward manifestation of a more fundamental uniformity in tlie con- 

 stitution of ova, and herein lies its significance. 



From this point of view, it is not the rule so much as the exception 

 which stands in need of explanation. In a spherical ovum developing 

 under normal conditions, there are no mechanical causes interfering 

 with the ordinary course of events, in a manner to bring about the 

 substitution of an equatorial for a meridian cleavage. The polar con- 

 centration of the germinal material would plainly have the contrary 

 effect; and, when carried to the extreme seen in the teleostean ovum, 

 would present conditions that are not only favorable to the normal 

 position of the first cleavage-spindle, but actually unfavorable to any 

 other position. 



** Biitsclili. Studicn ii. d. ersten Entwicklungsvorgiinge der Eizelle, etc., 

 pp. 10-22, 1870. 

 * Cucullanus forms an interesting exception, according to Biitsciili, No. 44. 



