204 M. A. Stacker on the Development 



when examined nnder a high power, appear to he filled with 

 very fine granules, become perceptible in the egg. The proto- 

 plasm-spheres increase more and more, until at last they form 

 a continuous voluminous layer around the vitelline spheres, 

 which are greatly reduced in vohuiie (fig. Y.). At the same 

 time we have an eqmilly important process to mention, namely 

 the gradual dissolution of the nuclei contained in the spheres 

 of segmentation ; under the microscope we can very well 

 follow the breaking-up of the individual nuclei into a great 

 number of granides. 



With regard to the origin of the protoplasm -balls, which 

 gi-adually increase in the egp^, 1 agree with MetschnikofF: 

 I believe that these have separated from the large spheres of 

 segmentation ; only I may remark that by this I understand 

 not the protoplasm-mass occurring in the individual spheres, 

 but the protoplasm collected in the reservoir, which has sepa- 

 rated itself. An analogous formation, a separation of proto- 

 plasm, occurs also in many Gasteropoda, Ctcnophora, Pla- 

 nariae, &c.* It is possible that in these animals also the whole 

 process takes ])lace in the sume way as in Chthonius — namely, 

 that in them also a portion of protoplasm is preserved through 

 the whole course of segmentation in a central cavity, and 

 afterwards separated therefrom. 



With the sejDaration of the protoplasm the spheres of the 

 primary deutoplasm confined in the central cavity also come 

 into view, with their form indeed a little altered, but still quite 

 recognizable in their origin as primary deutoplasm-spheres. 

 These collect at the periphery of the egg, where they gradually 

 constitute an albuminous-looking layer composed of a great 

 number of small spherules (figs. VI. & VII., pd). This is the 

 same layer which was indicated by Metschnikoff f " as perhaps 

 a kind of embryonal envelope ;" with regard to its origin 

 MetschnikofF says nothing further. It seems to me improbable, 

 however, that this layer represents an embryonal envelope, 

 and, indeed, for the same reason which is given in passing by 

 MetschnikofF. What function pertains to this structure, which 

 is apparently constant in the Chernetidse, is partially revealed 



♦ See Ray Lankester, " Observations on the Development of the Pond 

 Snail," Quart. Journ. Micr. Sci. vol. xiv. 1874; Carl Rabl, " Die Ontogenie 

 der Siisswasser-Pulmonaten," Jen. Zeitschr. I'iir Xatiirw. Bd. ix. (1875) 

 pi. vii. ; W. Fleniming, " Studien in der Entwicklungsgeschichte der 

 Najaden,"' Sitzungsb. Wien. Akad. Bd. Ixxi. (1875); A. Agassiz, 'Embry- 

 ology of the Ctenophora,' Cambridge, Mass., 1874 : and A. Kowalevsky, 

 " Embrvologische Studien an Wiirmern und Arthropoden," M^m. Acad. 

 St. Petersb. torn. xvi. (1871). 



1 MetschnikofF, " Entw. des Chehfet;" /. c p. 216, pi. xxxviii. figs. 7 & 

 8, ai. 



