OF ARTS AND SCIENCES. 37 



the first cleavage divides the ectodermic from the entodermic pole, as 

 supposed by Van Beneden, it is most probably equatorial. If further 

 researches prove that the cleavage begins with an equatorial groove, 

 a very important exception to the general rule will be established. 

 If, on the other hand, it turns out that the first cleavage is meridian, 

 it will be dilficult to reconcile this fact with Van Beuedeu's opinion 

 on the destination of the first two cleavage-spheres. 



The nematode ovum presents some difficulties, the importance of 

 which we would not underestimate. Goette^'" has followed the cleav- 

 age with such accuracy and detail that he feels warranted in asserting 

 that the first cleavage divides the ovum into two parts loliich shoio no 

 regular differences of size, form, or color, but which must nevertheless 

 be regarded as unlike in character, since one gives origin to the ecto- 

 derm and the other to the entoderm. Goette gives no information in 

 regard to nuclear transformations, and nothing definite on the polar 

 globules further than that they are first seen at one (aboral) end of 

 the ovum, and that they change position so as to be of no use in orien- 

 tation. The long axis is regarded as the axis of the ovum, and the 

 first cleavage as equatorial, since it cuts the axis at right angles near 

 the middle. Unlike most observers, Goette does not overlook the 

 fact that this is an exception to the general mode of cleavage among 

 the vermes ; and he explains it as the result of the confinement of the 

 ovum in a stiff ellipsoidal membrane, which prevents tlie ovum from 

 elongating transversely, as it would naturally do if the cleavage plane 

 coincided with the long axis (1. c, p. 65). Goette's observations form 

 a most valuable contribution on the promorphology of the vermian 

 ovum. He recognizes a homolosrous axis, an ectocfermic and an ento- 

 dermic pole, and uniform relations between the axis of the ovum and 

 that of the embryo, and between the poles and the dorsal and ventral 

 surfaces. All these relations become evident in the development of the 

 nematode, the moment we take account of the fact that the cleavage 

 process, in adapting itself to the form of the egg-membrane, causes 

 the ovum to rotate on its transverse axis throusjh 90°. 



The very interesting discovery made by Auerbach,*' that the pro- 

 nuclei perform a rotation of 90° on an axis perpendicular to the longi- 

 tudinal axis of the ovum, just before the cleavage begins, confirms the 

 opinion that there is a transposition of cleavage-planes, at least in the 

 case of Rhahditis nigrovenosa and some other nematodes. 



*2 Goette. Abliandl. z. Entw'gesch. d. Tiere, Erstes Heft, p. 59. Leipzig, 

 1882. 

 *3 Auerbach. Organologische Studien, II., p. 212, 1874. 



