INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 235 



in tlae green zone, decreasing rapidly with blue-violet, and reaching 

 its minimum in the red region of the spectrum. 



These facts establish the principle of the method, whilst they 

 explain the indifference which some invertebrates show to certain 

 rays (as red, &c.). The author is engaged on several points which 

 require further study : as to determine the rapidity with which the 

 retinal excitation disappears, and to estimate precisely the degree 

 of mouochromatism of the incident light ; this last condition would 

 only be incomjiletcly realized by employing media — coloured glass, 

 absorbent solutions, &c. — of which use has been made in the gene- 

 rality of experiments, and it seems rather as if it ought to be obtained 

 by the application of new processes, such as polarized light, &c. 



MoUusea. 



Development of Pulmonata.* — A long paper on this subject, 

 dealing chiefly with the ontogeny of jPlanorhis marginaius and 

 P. carinatus, is contributed by Carl Eabl. 



First period. — The formative or animal half of the egg contains 

 small yolk-granules, the nutritive or vegetative half large ones. Tho 

 first and second cleavage planes are meridional, including both animal 

 and vegetative poles ; the third is equatorial, at right angles to tho 

 other two, and divides tho egg into four small animal cells (micro- 

 meres), and four large vegetative cells (macromeres). A second 

 equatorial furrow then separates off from the latter four more micro- 

 meres; the embryo being now 12-celled. Next, each cell divides 

 into two ; the products of division of the eight micromeres have all 

 tho character of " animal " cells, that is, are finely granular : of the 

 eight cells formed by the division of the four macromeres, four, 

 situated near the equator, arc also animal cells, the remaining four, 

 alone of the twenty-four, retaining the character of vegetative cells, 

 that is, being bright yellow in colour and coarsely granular. At this 

 stage there is a largo cleavage cavity. 



Thus cleavage proceeds according to the numbers 4 ... 8 ... 12 

 . . . 21, or first in arithmetical, afterwards in geometrical pro- 

 gression. This Rabl states is the case in Gasteropods with a compa- 

 ratively small amount of food material in tlio egg (Dcrmatobranchiata, 

 Pulmonata), while it takes placo entirely by arithmetical progression 

 (4 ... 8 ... 12 ... IG ... 20 .. . 24) in tliose with a great amount of 

 food material (Pteroi)oda, Pleurobrauchiata, Ilcteropoda, and most 

 Profiobranchiata). 



Tho next important change is tlic division of tho four largo 

 vegetative cells, by whicli at last twelve cells, all having a macro- 

 meral character, are produced. Two of these become grown over, 

 and puslicd into tho cleavage cavity ; tli(>y aro tho jirimary iiu^soderra 

 colls. The remaining ten macromeres form the endodcrm, tho micro- 

 meres, now increased to alxmt forty, the ectoderm. So that in tho 

 .52-celled stage, tho three primary gorni-layers are already constituted. 

 Moroovor, tho embryo exhibits true bilateral symmetry, tho two 



» 'Morphol. Jahib., v. (1879) p. 562. 



