252 



I. IKEDA: 



me to give up my studies long before obtaining any hint towards 

 elucidating the question. The segmentation I have been able to 

 follow up only as far as the 64-cell stage, in which no directive cleavage 

 determining the future body-axes has as yet set in. As to the general 

 mode of segmentation, the present case belongs to the perfectly 

 spiral type which prevails in some other nemertines as well as in 

 polyclads, polychaetes, echiuroids and molluscs. The process pro- 

 ceeds to the 28-cell stage in much the same way as was described 

 by E. B. Wilson 1 ' for Cereb nitidus lacteus and by Zeleny 2 ' for Cere- 

 b nitidus marginatus, except in the fact that the first dexiotropic cleav- 

 ages giving rise to the 8-sell stage are equal instead of being 

 unequal. In the accompaning table is given the order of cell-division 

 up to the 64-cell stage, dt. and // being used to denote dexiotropic 

 and lacotropic divisions. The sign = denotes equal division ; _]_ or ~[~, 

 unequal division ; and Jl or TT, markedly unequal division of blasto- 

 meres giving rise to the upper or lower larger cells. 



As in Arenicola cristalli studied by Child, 35 the rosette cells (ia m , etc. 

 in textfigs. I and II) are first formed at the 36-cell stage by remarkably 

 unequal lacotropic divisions of the cells ia n , etc, of the 32-cell stage 

 into ia m and ia m , etc. The 44-cell stage is reached by two simulta- 

 neous equal and lacotropic divisions of ia n , etc. and ia t , , etc. of the 

 first quartet into ia < n and \a iv , ia M and ia 21 , , etc. etc. (see text 

 figures I and II). After that, the 48-cell stage is brought about by 

 equal and lacotropic divisions of ia r . , etc., which correspond to the 

 intermediate cells of polychaete-eggs, into ia ia and ia m , e tç. The 

 cells \a vn , etc. are situated each at the apex of the cross-figure formed 

 by the rosette cells (\a m etc.). A remarkably unequal cleavage is seen in 



1) Wilson, E.B., Experiments on Cleavage and Localization in the Nemct'tine Egg. 

 Arch. f. Entw.-Mech., Bd. 16, 1903. 



2) Zeleny, Ch., Experiments on the Localization of Developmental Factors in the 

 Nemertine Eggs. Journ. Exp. Zool., Vol. I, 1904. 



3) Child, CM., The Early Development of Arenicola and S/emasfis. Arch. f. Entw.- 

 Mech., Bd. 9, 1900. 



