224 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
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 Guignard 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 hemisphere, the spermatazoón 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 II. 
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). 
1 Boveri (95), in a paper recently received, completely confirms the observations 
of Wilson and Mathews regarding the source of the attractive bodies of the first 
cleavage spindle of the sea-urchin 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 the 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-haematoxylin, I have not been able to detect in 
the egg of Ciona; nor have I observed a substance (Doveri'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. 
