224 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



and in others by the ovum only, it is not inconceivable that in yet other 

 cases both may contribute to its formation. Therefore the observations 

 of Giiignard and Conklin are not necessarily irreconcilable with those 

 more recently made by Wilson and Mathews, Mead, Wheeler, and my- 

 self, as well as the earlier observations of others. In any case, however, 

 the theoretical conclusions based on Fol's " quadrille," as to the share 

 which the attraction centres enjoy in the phenomena of heredity, may 

 now be definitely set aside. ^ 



V. POLARITY OF THE EGG. 



Attention has already been called to the fact that even before fertiliza- 

 tion one axis of the egg, the vertical, has been determined. The point 

 where the polar globules form is its dorsal pole, which lies at the centre 

 of the surface of the less richly protoplasmic hemisphere. At some point 

 on the surface of the opposite hemisphei'e, the spermatazoon usually enters 

 the egg, and there is reason to believe that its point of entrance deter- 

 mines the median plane of the embryo, and so its antero-posterior axis. 



After the two pronuclei have met, they move toward the centre of 

 the egg, and in that region the first cleavage spindle arises (Plate III. 

 Fig. 14). It invariably lies parallel to a tangent at the point of forma- 

 tion of the polar globules. The first cleavage plane, which in accord- 

 ance with a general law is perpendicular to the spindle at its equator, 

 passes through the point where the polar globules arose and divides the 

 egg into two equal blastomeres (Plate III. Fig. 15 ; cf. Plate V. Fig. 27). 



^ Boveri ('95), in a paper recently received, completely confirms the observations 

 of Wilson and Mathews regarding tlie source of the attractive bodies of the first 

 cleavage spindle of the sea-urcliin egg. He for the first time in his published writ- 

 ings, so far as I know, gives a formal definition of the centrosome, applying the 

 term to what Wilson and Mathews call the " archoplasm." Boveri, if I rightly 

 understand him, recognizes an archoplasm surrounding tlie centrosome, at least at 

 certain stages, and specifically different both from the centrosome and from the 

 general cytoplasm. 



What in the foregoing pages I have called indifferently archoplasm and attraction 

 sphere undoubtedly corresponds with what Boveri in his latest paper ('95) defines 

 as the centrosome. A centrosome in the sense of Heidenhain, that is, a simple, dis- 

 tinct granule staining black in iron-litematoxylin, I have not been able to detect in 

 the egg of Ciona; nor have I observed a substance (Boveri's archoplasm) specifi- 

 cally distinct from the egg cytoplasm, enveloping the attractive body (Boveri's 

 centrosome). As the reader will glean from the earlier pages of this chapter, I 

 regard the substance forming the radiations about the attractive body as identical 

 with the egg cytoplasm. — June, 1895. 



