142 ADELE M. FIELDE. 



has entered the cytoplasm of the egg forms a deeply staining 

 rounded mass in strong contrast to the remaining portions. Sur- 

 rounding it is an area of cytoplasm staining less deeply than other 

 portions of the egg — indicated in the figure by the dotted line. 

 The appearance of the head-like structure is all the more remark- 

 able since no trace of anything of the kind is visible before en- 

 trance. Evidently the anterior end of the spermatozoon loses its 

 greatly elongated form after it enters the egg. It has been im- 

 possible to determine how much of the spermatozoon is involved 

 in this change and also whether the remaining portions, if any, 

 fuse with the cytoplasm or are cast off. 



It is impossible to distinguish with certainty the male pronu- 

 cleus from small yoke-spherules after the "tail" of the sperma- 

 tozoon disappears. The earliest stages observed with anything 

 like certainty are shown in Figs. 2, 3 and 7 (PI. II.). By the time 

 the egg reaches the stage of the second polar spindle, however, 

 the male pronucleus can usually be found in some part. In the 

 eggs shown in Figs. 9 and 10 (PI. III.) it was present in other 

 sections: 



By the time maturation is completed the male pronucleus has 

 attained large size and shows a faintly staining reticulum with a 

 nucleolus of large size. Figs. 11-14 (PI. III.) show the two pro- 

 nuclei at various stages of approximation to each other. 



It has not been possible to obtain the slightest evidence in sup- 

 port of the view that the cleavage centrosomes arise from the 

 spermatozoon. In the early stages of the male pronucleus (Figs. 

 2, 3 and 7, PL II.) no trace of spheres or centrosomes has been 

 observed in connection with it. In a number of cases the two 

 centrosomes have been observed lying near the pronuclei before 

 cleavage (Figs. 12, 13, 14, PI. III.) but in no case was there the 

 slightest indication that they were more closely associated with 

 one nucleus than with the other. 



There can be little doubt from these observations that the pe- 

 culiar spermatozoa of Moniezia actually fertilize the eggs and 

 therefore that they contain nuclear substance in some form or 

 condition, or at least substance capable of giving rise under proper 

 conditions to a nucleus. If future investigation shall establish 

 what seems at least possible from my own observations, viz., that 



