■{54 Naoliide Yatsii. 



4. Rays 387 



r». Cell c'oiistructiou and uiid-body o87 



b. General remarks 388 



VI. Formation of the polocytes , 392 



VII. Summary 393 



VIII. Literatm-e 396 



I. Introduction. 

 It is nine years since Coe ('99) wrote bis excellent paper on the 

 maturation and fertilization of the egg of Cerebratulus marginatus 

 Renier. The remarkable advance of our knowledge of the achro- 

 matic structure of the cell attained during this time has made it 

 desirable to re-examine some of the most important phenomena in 

 the nemertine egg; for it is one of the most favorable objects for 

 cjtological study. Five years ago Professor Wilson suggested to me 

 to make a careful study of maturation, fertilization and early 

 cleavage stages of the egg of Cerebratulus lacteus, partly in order 

 to clear up some disputed cytological problems and partly to give a 

 morphological basis for my work on experimental cytology and em- 

 bryology. 



The materia], from which the results of the present paper were 

 obtained, consists of several complete series of the early develop- 

 mental stages of the egg of Cerchraluhis lacteus put up by Professor 

 Wilson and Dr. Sutton in the summer of 1902, at South Harps- 

 well, Maine. A few lots treated with some salt solutions were fixed 

 by the writer, and were made use of for the sake of comparison. To 

 supplement the general discussion a few results obtained from the 

 study of the e^g of Asterias forhesii have been incorporated.^ 



II. Structure of the Egg. 



(a) Cytoplasm. 

 The cytoplasm of the egg of C. lacteus, as in maiiy other forms, 

 shows an alveolar structure, consisting of fine yolk drops of fairly 



-I am under sreat obligation to Dr. H. M. Smitli and other members of the 

 Biological Laboratory of the United States Fisli Commission at Wood's Hole, 

 where my cytological woi-k on the starfish egg was partly carried out in the 

 sunnner of 1902. 



