344 SUMMARY OF CURRENT RESEARCHES RELATING TO 



are found in the hind-brain opposite the otic invaginations, in the nuclei 

 and mot fibres of the cranial nerves, and in a centre on either side of 

 the extreme anterior end of the mid-brain. 



There is no evidence that mitochondria are transformed into neuro- 

 fibrils. The mitochondria do not decrease in amount as the neurofibrils 

 develop, nor change in their structure or staining; reactions. Mitochondria 

 are present in the early stages in the differentiation of the nerve cells of 

 the chick, and there is ample evidence that they occur throughout 

 cytomorphosis. The neurofibrils, on the other hand, are only present 

 from the fifteen-somite stage onwards, in evident adaptation to functional 

 demands. Mitochondria may therefore be regarded as cytoplasmic 

 elements of a generalized nature, not participating in so specialized a 

 cell function as the development of neurofibrils; while the neurofibrils 

 are to be looked upon as indicative of the differentiation of the cells in 

 which they are found. 



Development of Rectal Gland of Squalus acanthias.* — E. R. 

 Hoskins finds that the " digitiform gland " appears as a slight thickening 

 of the endoderm of the dorso-lateral border of the gut just posterior to 

 the spiral valve. The thickening soon pushes out laterally to form a 

 hollow bud which turns and grows anteriorly along the gut. From the 

 main part of the gland small buds resembling the original form of 

 the gland grow laterally on all sides ; they become tubules which give 

 rise to secondary tubules ; the gland becomes a compound tubular 

 structure. As the gland develops it carries the mesentery of the 

 intestine with it, and is thus supported from the dorsal wall of the body 

 cavitv. The endoderm is at first four layers thick ; it is gradually 

 reduced to a single-layered epithelium from which the primary and 

 secondary tubules grow out. The epithelium lining the main or central 

 lumen subsequently thickens, resulting in a structure of two layers of 

 columnar cells with rounded nuclei in the embryo before birth, and of 

 four layers in the adult fish. 



Development of Lungs of Alligator.f — A. M. Reese finds that the 

 primordia of the lungs in the alligator are budded off from the ventral 

 side of the pharynx, just behind the region of the gill clefts. They are 

 first seen in embryos of about thirty somites. A description is given of 

 the appearance and minute structure of successive stages, and there are 

 many illustrations. 



Reaction of Embryonic Cells to Solid Structures.^ — Ross ,; - 

 Harrison has tested the hypothesis that embryonic cells moving in 

 clotted lymph are positively stereotropic. The plan of experimentation 

 consisted in varying the three main factors involved in the cultivation 

 of tissues, viz. the tissue itself, the fluid medium, and the solid support. 



* Proc. Amer. Assoc. Anatomists, 31st Session, in Anat. Record, ix. (1915) 

 pp. 83-1. 



t Smithsonian Misc. Coll., lxv. (1915) pp. 1-11 (9 pis.). 

 X Journ. Exper. Zool., xvii. (1914) pp. 521-44 (14 figs.). 



