Y. — Reversal of Cleavage in a Sinistral Gasteroi)od. 



BY HENRY E. CRAMPTON, JR. 

 Read Feb. 12, 1S94. 



The purpose of the present preliminary notice is to communicate 

 an interesting series of embryological facts which have not hereto- 

 fore been published. They are the results of studies made during 

 last December upon the cleavage of the eggs of our common fresh- 

 water Pulmouates, Physa heterosti-opha, and Limnaea columella. 

 Of these it will be remembered that the adult Physa has a sinistral 

 shell, and Limnjea a dextral shell, the more usual form. 



It was by a most happy accident that these facts were brought 

 to light. At the suggestion of Prof. E. B. Wilson, I was about to 

 study the development of Paludina, which, according to von Erlan- 

 ger, forms the mesoderm by archenteric pouches. As the Pulmo- 

 uates were commoner at that time of the year, the idea occurred to 

 me to study their development in order to have a more thorough 

 basis for comparison, when I came to study Paludina. But it was 

 with the full expectation of finding the usual method of cleavage, 

 and not with any idea of the presence of variation. 



Owing to the short time of my investigation I have demonstrated 

 few stages beyond the formation of the mesoderm, which takes place 

 at the 28-cell stage. But the early stages have been most thoroughly 

 determined, and these I shall describe. I emphasize the point that 

 these are the determinative stages, in which the orientation of the 

 animal is completed, and are assuredly the most important. 



The type of cleavage in these eggs is the spiral, well known in 



many forms, notably Nereis (Wilson, Journ. Morph., vol. vi. No. 3), 



Umbrella (Heymons, Zeit. f. wiss. Zool., Band 57, 1893), Neritina 



(Blochmann, Zeit. f. wiss. Zool, Band 36, 1882), Crepidula (Conk- 



AxNALS N. Y. Acad. Sci., VIII, March, 1894.— 12 



