40 SUMMARY OF CURRENT RESEARCHES RELATING TO 



organism plastic factors which from a homogeneous cell material may 

 mould differently organized products. On the basis of descriptive 

 histogenetic studies it seems plausible to admit that environment can 

 modify isolated cells ; that the metabolic processes of the cells are the 

 resultant of their physico-chemical constitution plus physico-chemical 

 conditions of the environment (including hormones, enzymes, and so 

 forth), and do not depend exclusively upon their physico-chemical 

 constitution ; that different substances arise in the cell-body (haemo- 

 globin, various specific granules) in polyvalent cells as the result of 

 changes, determined by differences in the environment. 



Behaviour of Explanted Striped Muscle in Cultures.* — Warren 

 H. Lewis and Margaret R. Lewis have experimented with muscle frag- 

 ments explanted from a chick embryo and placed in tissue cultures like 

 Locke's solution. The muscle-buds show de-differentiation or return to 

 a more embryonic condition. The de-differentiation never proceeds to 

 an indifferent stage. The initial stages in regeneration of muscle in 

 mammals and amphibians are much like those observed in these culture- 

 experiments. In both there is (1) a formation of young myoblasts, a 

 return to a more embryonic condition ; (2) the formation of proto- 

 plasmic buds which grow out from the ends of the old fibres. Such 

 buds contain many nuclei and lack cross-striations. Their formation is 

 inherent in the muscle-fibre itself, and becomes manifested when; the 

 fibre is cut across or injured. Although the initial stages are much the 

 same in cultures and in ordinary regeneration, it is not to be expected 

 that even after prolonged cultivation m vitro there will be a re-differen- 

 tiation of the muscle-buds. This requires more complex developmental 

 conditions. It is interesting that anastomosis of muscle-buds may occur 

 in cultures. 



Thymus-like Structures in Larval Lamprey.t — Ivan E. Wallin 

 finds that the lymphocyte accumulations, or placodes, in the lateral 

 branchial wall of the larval lamprey do not represent primitive thymus 

 primordia. Similar formations are present in other parts of the 

 pharyngeal epithelium. An important component of the thymus of 

 higher animals is a reticulum ; there is no indisputable evidence of a 

 reticulum in the placodes. They are apparently patches of degenerating 

 epithelium. 



The branchial " epithelium " does not represent a pure endodermic . 

 epithelium. This " epithelium " devotes hsemopoetic properties in the 

 advanced larva. " Epithelium " from the gill arches invades the 

 ciliated epithelium of the epipharyngeal ridge and produces placodes. 

 These placodes have a relationship to the gill arches and gill pouches 

 which make them homologous with the thymus placodes of Elasmo- 

 branchs and are to be considered primitive thymus structures. The 

 lymphocyte-like cells which originate in the primitive thymus placodes 

 differ in structural characters, and in mode of origin from the lympho- 

 cytes which are formed in the gill arches and lamellae. 



♦ Amer. Journ. Anat., xxii. (1917) pp. 169-94 (14 figs.). 



t Amer. Journ. Anat., xxii. (1917) pp. 127-67 (4 pis. and 3 figs.). 



