ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 245 



Development of Mucous Membrane of Stomach.* — Maurice de 

 Laet has studied this in embryos of the rabbit, and finds that the 

 stomach grows by its epithelium, the mesoderm only following this 

 growth and adapting itself thereto. Processes of vacuolization, growth 

 of epithelium, formation of primary villosities, and transformation into 

 secondary villosities occur in a fashion analogous to that described by 

 Tandler for the duodenum, and by Kreuter f or the oesophagus. But the 

 epithelial proliferation never amounts to a transitory obliteration of the 

 lumen of the stomach. Increase " by vacuolization " is characteristic of 

 the narrower and more muscular regions of the alimentary canal. At 

 first the epithelial proliferation appears to be of equal intensity all over, 

 and it seems everywhere to secrete, or at least to allow the filtration of, 

 a fluid charged with mucus. In the fundus the musculature, reduced 

 in the adult, rudimentary and slow of development in the embryo, offers 

 hardly any resistance, and readily allows the epithelium to extend. In 

 the oesophagus and duodenum, and in the pyloric region, it resists more 

 and thus modifies the intra-epithelial processes. But this is the limit of 

 the role of the mesoderm in the development of the stomach. It has 

 but a mechanical influence, supporting the epithelial layer that covers it. 

 The latter develops of itself in virtue of its inherent properties. 



b. Histology. 



Tadpole Epithelium cultivated in Plasma.f — S. J. Holmes has 

 studied the behaviour of the ectoderrnic epithelium of tadpoles of 

 Rana, Hyla, and Diemyctylus, the tissue being kept alive in culture 

 media, such as blood-plasma and lymph. Pieces of tissue frequently 

 send out extensive strands or sheets of ectoderrnic cells into the sur- 

 rounding medium. The cell masses tend to extend upon some solid 

 surface such as the cover-slip, fibres of cotton, or the surface of a 

 coagulated mass of plasma. The isolated cells of ectoderrnic epithelium 

 show a tendency to spread out widely and to creep along a solid surface. 

 Scattered cells which come into contact in the course of their spreading 

 may form a perfectly continuous secondary membrane. Active epi- 

 thelial cells have a thin clear margin of protoplasm which puts forth 

 fine pseudopods and undergoes marked changes in outline. It is 

 probably by means of this amoeboid movement that the spreading and 

 migration of individual cells are accomplished. Evidence from several 

 cases was obtained to show that the masses of cell that were put forth 

 were not pushed out and did not grow out, but were drawn out by the 

 amoeboid activity of the cells at the free surface of the mass. Un- 

 favourable conditions tend to make the epithelial cells assume a rounded 

 form. Heat causes them to retract their pseudopods and makes the 

 strands of cells contract and break up into masses. There is a limited 

 amount of multiplication in the isolated cells of the ectoderm. Amitotic 

 divisions of the nucleus may occur in vitro, but they were not found to 

 be followed by a cleavage of the cytoplasm. Isolated cells from young 



* Arch. Biol., xxix. (1914) pp. 353-87 (1 pi. and 1 fig.). 



+ Public. Univ. California (Zool.) xi. (1913) pp. 155-72 (2 pis.). 



