ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 37 



from the mesial and primitive portion of the tectum opticum, con- 

 stricted off, and, as it were, left behind in the enormous development 

 of the tectum in this aberrant group. 



In its early development in the ganoids it is the result of purely 

 mechanical causes, the rapid growth in the adult of the "nucleus 

 magnocellularis " producing a downward bending of the mesencephalic 

 roof on either side of the median plane. 



Mechanical causes are still plainly operative in the Siluridre, but in 

 other Teleosts the torus appears, at an early stage of ontogenetic 

 development, as the result of phylogenetic causes. Though the torus 

 longitudinalis is a structure which first attains an independent and 

 definite form in the Teleosts, and in that group only, its essential 

 elements are perhaps the most archaic of the mesencephalic roof. The 

 structure and functions are also discussed. 



Development of Body Cavity and Gonads in Salmonidae.* — U. Bold 

 has studied this in trout and salmon embryos. He finds that its first 

 appearance in the trout is on the 25th day after fertilisation. It arises 

 as a cleavage between somato- and splanchnopleure in the region of the 

 lateral head plate. On the 28th day the body cavity has increased in 

 the cranial, and especially in the caudal direction ; its development is 

 closely connected with that of the gut. At :58 days, growth in length 

 has ceased, whilst lateral development, coincident with yolk absorption, 

 has considerably increased. In salmon embryos of 40 days (twenty 

 segments) the body cavity of the tail is completely separate from that 

 of the yolk-sac. Later the yolk-sac body cavity in the region of the 

 14th to 18th trunk segments is drawn into the trunk body cavity ; the 

 yolk-sac, which has collapsed from the 18th segment onwards, forms for 

 a time a long mesenteric formation, through which the ventral body 

 partition is joined to the ectoderm. Later it is completely absorbed. 



In trout the first genital cells were not found before the 25th, and 

 in salmon the 31st day. There are two stages of development of the 

 genital organs. The first, that of the genital ridge, was observed in 

 the salmon on the 60th day. The second, that of the genital fold, 

 arises from the ridge, and may be recognised in the salmon on the 

 85th day. In its anterior part it never extends beyond the 4th, and 

 in its posterior or caudal, never beyond the 38th. Three kinds of cells 

 arise from ccelome >cells, viz. indifferent, follicle, and genital cells. 



Relation of Nervous System to Developing Musculature.f — 

 R. G. Harrison has investigated this subject experimentally. The 

 spinal cord of embryos of Rana was removed before histological dif- 

 ferentiation in either muscular or nervous tissue had begun. This did 

 not hinder the differentiation of the contractile substance in the normal 

 manner, nor the grouping of the individual fibres into muscles. Larvse 

 were reared under continued narcosis of acetone-chloroform, which stops 

 all voluntary movements, including those of respiration, while the heart- 

 beat is scarcely affected. The functions are rapidly restored by removal 

 from the drug. Larvas reared in this way and imbedded side by side 



* Morphol. Jahrb., xxxii. (1904) pp. 505-86 (1 pi.). 

 t Amer. Journ. Anat., iii. (1904) pp. 197-220. 



