446 SUMMARY OF CURRENT RESEARCHES RELATING TO 



definite neuromeres, whose number is constant throughout the series, 

 and whose enlargement and fusions have directly accompanied regional 

 differentiation, but tbat it represents a complex of colonies of nerve- 

 cells whose elements having similar functions are gradually drawn from 

 various body-regions to centres whose ultimate position is a matter of 

 physiological convenience. It is the physiological convenience, accord- 

 ingly, which might be looked upon as the determinant in matters of posi- 

 tion, and it is this which becomes the criterion of the homology of parts. 



It should be noted, in conclusion, that the above citations and sum- 

 mary cannot suggest the detailed descriptions which form the bulk of 

 this sumptuously illustrated memoir. 



Development of Amphioxus.* — Prof. E. W. MacBride returns to 

 the much disputed question of the origin of the atrial chamber in 

 Amphioxus. He finds himself in agreement with Kowalevsky as to 

 the existence of atrial ridges, which appear at an early stage of develop- 

 ment, and contain each a cavity which is an extension of that of the first 

 myotome (collar-cavity). The ectoderm on the outer side of these ridges 

 becomes thickened, and ultimately hollowed out to form the lymph 

 canals, which the author no longer regards as of ccelomic origin. The 

 thickenings and the contained canals are metapleural. The (ccelomic) 

 cavities of the atrial ridges become later separated from the collar- 

 cavities, and constitute the metapleural coelom at each side ; later this 

 becomes filled up by cells. The atrial ridges unite beneath the ventral 

 surface of the body, and enclose the atrium. The upward growth of the 

 atrial chamber described by Lankester and "Willey does not occur, the 

 appearances noted by these authors being due to the disproportionate 

 growth of the ventral part of the pharynx of the larva. 



The paper, which is controversial in tone, includes a discussion of 

 various criticisms passed on the author's previous paper. 



Development of Nasal Groove. f — Dr. Karl Peter has tried to dis- 

 cover whether the invagination is due to a simple folding without 

 localised growth, or is the result of local proliferation. His method 

 involved much careful enumeration and observation of mitosis. There 

 seems no doubt that the development is due to the proliferation of a 

 localised growth of cells, and not to the folding of a uniformly growing 

 plate. The mesoderm does not seem to have any influence ; it is a 

 wholly ectodermic affair ; nor does the brain affect the growth. Besides 

 mechanical conditions, there are favourable nutritive conditions to be 

 considered, which enable the cells on the lumen side of epithelial organs 

 to increase with special rapidity. 



7. Histology. 



Structure of the Cell4 — Dr. Nils Sjobring argues in support of the 

 idea that the cell includes two sets of structural complexities : — (a) the 

 trophoplasm, having to do with nutritive metabolism, and (&) the kino- 

 plasm, concerned with motor processes. The two are apposed, not topo- 

 graphically sepai ..ted. Like the nucleus, both are under the control of 

 the archiplasm. The fundamental structure consists of threads and rows 



» Quart. Journ. Micr. Sci./xliii. (1900) pp. 351-66 (1 pi.). 

 t Arch. Mikr. Anat., lv. pp. 585-617 (1 pi. and 5 rigs.). 

 X Anat. Anzeig., xvii. (1900) pp. 273-304 (3 figs.). 



