3G PROCEEDINGS OP THE AMERICAN ACADEMY 



111 the rotifera, according to Salensky's ^ observations on Brachio- 

 nus nrceolaris, the first cleavage-plane is equatorial ; and this is 

 followed by two meridian cleavages of the smaller sphere. Had 

 Salensky succeeded in finding polar globules and in tracing circum- 

 stantialhj the origin of the 5-celI stage, we could have no hesitation in 

 conceding the full force of the exception. The eggs of the rotifera 

 are by no means favorable objects for deciding the question under 

 consideration. No satisfactory evidence of polar globules in tliese 

 ova has been obtained. If these ova develop partlienogenetically, as 

 supposed by Cohn^®and Huxley^'; and if, on this account, no polar 

 globules are produced, it might be possible to explain the appearance 

 of an equatorial cleavage prior to the meridian planes of cleavage. 

 The archiamphiaster which gives rise to the polar globules always 

 lies in the axis of the ovum, and its plane of division is therefore 

 parallel to the equator of the ovum. If the division of the archiam- 

 phiaster led to the division of the ovum, instead of the formation of a 

 polar globule, the first cleavage-plane would be equatorial or parallel. 



Ill regard to the gephyrea, Ilaeckel is certainly in error, in proof of 

 which we may refer to the investigations of Selenka^^ on Phascolo- 

 soma, and of Spengel ^ on Bonellia. A comparison of Selenka's 

 figs. 1 and 2, PI. XXIX., with Ilaeckel's figs. 93 and 94, PI. XXIV., 

 will show how easily a meridian cleavage, where the polar globules 

 have not been observed, could be mistaken for an equatorial one. 



The researches of Van Beneden ^° on the maturation, fecundation, 

 and cleavage of the mammalian ovum, of which we have thus far re- 

 ceived only a preliminary account, leave it doubtful whether the first 

 cleavage-spindle is parallel with the axis of the ovum or at right 

 angles to it. He calls attention to the pronounced polarity of the 

 ovum, but expressly states that he has not yet obtained a complete 

 history of the first cleavage-spindle.^^ The relation of the plane of 

 first cleavaije to the axis of the ovum is therefore undetermined. If 



85 Salensky. Zeitschr. f. wiss. Zool., XXII., p. 45G, 1872. 



86 Cohn. Zcit.schr. f. wiss Zool., VII., 1856. 



87 Iluxlcy. Trans. Micr. Soc, 1853. 



^ Solenka. Zeitschr. f. wiss. Zool., XXV., p. 444, 1875. 



89 Spengel. Mitt. a. d. Station z. Neapel, I., p. 374, 1879. 



*0 E. van Beneden. "La Maturation de rO'.iif, la Fccondation ct les pre- 

 mieres Pliases du Developpenient Einbr^onnaire d. Mammifercs." Bull, de 

 I'Acad. Roy. des Sci. de Belgique, December, 1875. Also in Journal de Zo- 

 ologie, v., 1876. 



*> III. "Formation des Feuillets cliez le Lapin." Arcli. de Biol., I., pp. 140, 

 141, 1880. 



