No. I.] THE EGG OF UMAX AGRESTIS. 225 



of the " archoplasm " in Physa depends merely on the way in 

 which the protoplasm is collected around the centrosome. 



The cytoplasmic nature of the "archoplasm" in Limax is 

 revealed by various changes that go on in the centrosphere 

 itself, rather than by changes in the surrounding cytoplasm. 

 PI. XI, Figs. 10 and 13, and PL XII, Figs. 22, 25, 27, 28, and 

 30, show successive stages in the disappearance of the " archo- 

 plasm " and of the " centrosome " of the Ascaris type. PI. XI, 

 Fig. 13, shows the centrosphere approaching the type of the 

 "attraction sphaere " of Van Beneden, or the " Mikrosphaere " 

 of Heidenhain. The outer "cortical" zone of the centrosphere 

 is bounded by a circle of granules (microsomes) in which the 

 radially arranged cytoplasmic fibers at first seem to terminate. 

 Within the centrosphere the astral rays are continued as 

 extremely delicate fibers which traverse the " cortical " zone 

 and terminate on the periphery of the " medullary " zone. The 

 central zone is uniformly finely granular in appearance. It is 

 without a distinct center and is not penetrated by the radial 

 fibers. In the second stage of growth of the centrosphere, 

 after the extrusion of the second polar globule, there is no 

 trace of a distinct homogeneous ("cortical") zone. The astral 

 rays come closer together as they approach the periphery of 

 the medullary zone or "heller Hof " (centrosome of Boveri), 

 but they always remain distinctly separate. They can be 

 traced into the reticulum which traverses the "heller Hof" 

 and which is attached to the centrosome at the center of the 

 sphaere. The " cortical " zone of the archiamphiaster stage 

 now corresponds to that part of the cytoplasmic reticulum 

 which immediately surrounds the " heller Hof," and which, by 

 the radial arrangement of its fibers, forms the central rays of 

 the aster. 



Moreover, the transformation of the aster into a spiral that 

 involves the cytoplasm of the entire cell after the extrusion of 

 the second polar globule leaves little doubt as to the cytoplas- 

 mic nature of the aster. As the spiral rays diverge from the 

 centrosphere, they gradually break up into the cytoplasmic 

 reticulum. During the period of the disappearance of the 

 egg-centrosphere the sphaere is still further resolved into the 



