224 SUMMARY OF CURRENT RESEARCHES RELATING TO 



and of the organ of Bojanus lias been already indicated. A number 

 of cells arranged round the rectum in vesicular form, afforded at tbe 

 end of the parasitic life an indication of the future heart, (g) The 

 single-layered embryonic mantle passes at the margin of the shell 

 into a zone of several layers of small cells which form the future 

 mantle. The compression of the large, cylindrical or conical, mantle- 

 cells into the mushroom-shaped body has been already referred to. 

 He confirms Braun's description of the origin of the permanent 

 shell. 



III. Com])arison loith other Mollusca. — In comparing his results 

 with the ontogeny of other molluscs, Herr Schmidt notes the im- 

 possibility of homologizing the so-called byssus-gland of the 

 Glochidium with the similarly named organ in many Lamellibranchs. 

 The former is a special larval organ, adapted to the mode of life, and 

 might, Herr Schmidt suggests, be conveniently designated " Kleb- 

 fadendriise." He observed what Braun had previously recorded, that 

 in the absence of fishes, the mature embryos may remain for weeks, 

 or even months, in the branchiaa of the mother mussel. He emphasizes 

 Schierholz's demonstration that the posterior ciliated patch could not 

 be the rudiment of the velum, and attributes the peculiar posterior 

 situation of the gills, the foot, and the alimentary canal to the great 

 development of the embryonic adductor. In summarizing the 

 peculiarities of Najad development, e. g. in the mantle, he lays special 

 emphasis on the completely ectodermic origin of the nervous system, 

 which does not readily harmonize with Hertwig's theory. 



Development of Vermetus.* — Prof. W. Salensky reports the 

 result of his study of the development of Vermetus. In the young 

 unfertilized ova a small " protoplasmic " and larger " deutoplasmic " 

 portion are readily distinguished; the segmentation resembles that 

 of other molluscs, the " micromeres " appearing at the formative pole 

 by separation of the "protoplasmic" portion of the " macromeres." 

 When sixteen of these have thus been formed the epibolic gastrula 

 formation begins ; after about two-thirds of the egg has been surrounded 

 by the slow multiplication of these cells a small hollow, representing 

 the archenteron, is formed on the ventral surface. The round blasto- 

 pore, at first in the centre of the ventral surface, gets shunted 

 gradually backwards, becoming oval, and ultimately forming the 

 mouth-opening. Meanwhile the macromeres are dividing, at first 

 posteriorly, and the small cells thus resulting form the future endo- 

 derm, and displace the undivided macromeres which go to form yolk. 

 The mesoderm appears much later, at first as a single layer of cells, 

 arising from the ectoderm round the rim of the blastopore, but 

 subsequently exhibiting several layers, splitting to form the body- 

 cavity, &c. 



The rudiment of the foot is seen at the time of mesoderm formation, 

 as an axial row of ciliated cells stretching from the blastopore back- 

 wards, and with slightly larger ectoderm cells on each side. A 

 similar ciliated ridge occurs at the cephalic portion of the embryo 



* Biol. Centralbl., v. (1885) pp. 564-8. 



