ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 735 



stage there is a larger and a smaller blastoraere on either side of the first 

 groove, the larger occupying the part in which were the polar globules, and 

 which may be called anterior or superior. In the third stage the superior 

 commence to divide before the inferior blastomeres, and, as this becomes 

 more marked, we get an intermediate stage with fourteen blastomeres. 

 When the sixteen appear, two kinds of elements may be distinguished ; 

 those which are separated and individualized the author proposes to call 

 " blastomeres," while the others, which are continuous at their periphery 

 with the unsegmented formative yolk, he calls " blastocones." In the fifth 

 stage the first segment (or what most authors call blastomere), counting 

 from above, divides across and gives rise to a blastocone and a blastomere, 

 the second divides longitudinally and forms two blastocones, the third 

 and sixth divide longitudinally, the second and fourth transversely. In 

 the next intermediate stages we have twenty blastocones and eight blasto- 

 meres, and later on, thirty blastocones and eighty blastomeres, owing to 

 the transverse divisions which produce the blastomeres having been more 

 frequent in later stages of segmentation. 



The segmentation of Cephalopods calls to mind that of other Mollusca, 

 the blastocones representing the macromeres, and being, like them, pro- 

 duced by a kind of budding from the blastomeres (which are the equivalent 

 of the micromeres) ; the incomplete separation of the former is regarded as 

 a secondary process. 



As the blastocones divide and form rays around the blastoderm the 

 elements produced by it separate from one another, their contour becoming 

 less distinct ; their protoplasm becomes hollowed out with vacuoles and 

 gradually diffuses into the hyaline layer, so that in place of an element of 

 definite form we only have a nucleus surrounded by a very small quantity of 

 granular protoplasm. The blastomeres, then, form a circular plate (blasto- 

 derm), which will give rise to the body of the embryo, while the blastocones 

 have formed a plasmodium which will become the perivitelline membrane ; 

 this last becomes intercalated between the blastoderm and vitellus, and in 

 time comes to separate them completely; it is not formed, as Prof. 

 Lankester thought, by vitelline nuclei, but arises from the first two 

 segmentation-nuclei. 



Development of Reproductive Organs in Gasteropods.* — Prof. C. 

 Semper subjects to a searching criticism the recent conclusions of Brock f in 

 regard to the development of the reproductive organs in Gasteropods. Brock's 

 conclusions were based on a study of Limax agrestis, and may be roughly 

 summed up as follows: — (1) At a very early stage the thin hermaphrodite 

 duct is connected with the genital atrium and associated penial diverticulum 

 by a somewhat thicker canal — the primary genital duct. (2) The vas 

 deferens arises as a blind canal from the end of the penis, while the primary 

 genital duct splits into two adjacent canals, the female and male ducts. 

 (3) The latter is a provisional and transient structure, and from the 

 remaining half there develope both oviduct and spermatoduct. (4) The 

 free vas deferens, arising as above noted from the penis, unites with the 

 female duct before it divides into oviduct and spermatoduct. (5) The 

 spermatheca arises as a fine diverticulum from the base of the penis, at 

 the point where the latter is inserted along with the oviduct in the genital 

 atrium. 



So much for Brock's conclusions, which Semper proceeds to criticize. 

 The male duct is said to disappear without trace, but of this no proof is 



♦ Arbeit. Zool.-zoot. Inst. Wuizburg, viii. (1887) p. 213. 

 t See this Journal, ante, p. 58. 



3 c 2 



