466 BULLETIN OF THE 



The eggs of the sea-urchin are peculiar from the great length of time 

 (sixteen to eighteen hours) daring which they remain capable of normal 

 fertilization. The abnormal penetration of several spermatozoa Hertwig 

 thinks is due to the protoplasm, impaired in its vital energies, no longer 

 offering resistance to such penetration. 



Among mollusks the eggs of Mytilus afforded excellent results, which 

 in the main so far corroborate the evidence of his other observations 

 that I confine myself to a few minor points. Before the first maturation 

 spindle has reached the surface of the yolk a corpuscle (sometimes 

 divided into halves) is seen at some distance from the spindle. He is 

 not quite certain, but inclines to the opinion that it consists of nuclear 

 substance, for it disappears some time after fertilization, i. e. before the 

 formation of the polar globules. It is interesting to observe that the 

 egg does not advance beyond the formation of the first maturation spindle 

 unless it is fertilized. Then, after fifteen minutes, the polar globules are 

 quickly formed (the second follows the first after twenty-five minutes), 

 and carry before them the double-contoured egg membrane. The spindle 

 becomes shortened and thicJcer before the globule is formed. A promi- 

 nence arises at the vegetative pole of the egg when the first cleavage 

 amphiaster makes its appearance ; it ultimately forms a part of the 

 greater (vegetative) segmentation sphere. 



The criticism I have made on the account given by Fol of the early 

 changes in pteropod eggs, simply from a comparison with the changes 

 which occur in Limax, is strengthened by the conclusions to which 

 Hertwig arrives from a study of mollusks more nearly related to those 

 investigated by Fol. It follows from Hert wig's observations on Tiede- 

 mannia Neapolitana and Cymbulia Peronii that the formation of polar 

 globules and of the egg nucleus takes place in essentially the same 

 manner as in Asteracanthion. The two polar globules are formed one 

 after the other, — not by the division of a single globule. The " Ver- 

 dichtungszone " of the maturation spindle may in Tiedemannia be seen 

 in the living egg as a row of short dark rods. 



He passes over the formation of the second maturation spindle by 

 simply saying that the spindle-half which remains after the second polar 

 globule is formed, completes itself again. The female pronucleus arises 

 as a chister of vacuoles. It is noticeable that in all the mollusks de- 

 scribed by Hertwig, except lamellibranchs, it remains very near the 

 animal pole of the egg just as in Limax, and that in all cases the 

 female pronucleus, unlike Limax, seems to exercise less influence on 

 the surrounding protoplasm than does the male pronucleus. 



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