ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 163 



in the third month, the inner portion appearing first, the outer slightly 

 later. From the third month onward the body of the pancreas is (wholly 

 or partly) prismatic in 90 p.c. of all cases. 



Development of Dentine in Mammalia.* — K. v. Korff finds that the 

 ivory cells do not share in forming the collagenous dentine ground- 

 substance, but that they develop, by means of a secretory activity, only 

 the tooth-fibres which keep open the nutritive canals, i.e. the dentine 

 canals. The dentine at first is not homogenous, but is composed of 

 fibrils of connective tissue of the tooth pulp. The fibrils radiate peri- 

 pherally towards the upper surface of the pulp, and become felted with 

 the fibrils of the basal membrane which lies upon the enamel-cells. 

 The basal membrane thickens by the addition of fibrils which are always 

 growing from the interior of the pulp. In the zone of the ivory-cells 

 the fibrils of the pulp pass into the spaces between them. They here 

 form intercellular collagenous strands which split into fibrils, and these 

 merge into the fibrils of the dentine ground-substance. 



Development of Lungs in Tropidonotus.f — J. J. Schmalhausen has 

 studied this in T. natrix. In the adult a small left lung (2-4 mm.) is 

 present, which has a typical snake's lung structure, and is connected 

 with a bronchus provided with cartilaginous rings. Both lungs arise 

 simultaneously, or almost so, but the right grows very much faster than 

 the left, whose growth extends throughout the whole embryonic period. 

 In connection with the strong development of the right lung, there is a 

 displacement of the gut to the left and ventralwards, besides a twisting 

 of the right lung upon its axis, so that the left lies upon the ventral 

 face of the right. Histological differentiation also proceeds more rapidly 

 in the right than in the left lung. All this is in accordance with 

 Mehnert's formula, according to which the origin of retrogressive organs 

 tends to be later, and their development and histological differentiation 

 is glower than in progressive ones. The latter, contrarywise, are subject 

 to a hastening of the processes of development, which expresses itself 

 in the earlier origin, in the more rapid succession of one histological 

 stage after another, and in the rapid increase in size of the organ. 

 The left lung in T. natrix is a good example of the greater variability 

 of a rudimentary organ, e.g. it exhibits variation in its rate of growth 

 and in its time of origin. 



Development of External Genitals of Sheep. f — J. Bohm finds that 

 the first appearance of the sexual organs in the sheep is in embryos 

 of 1 • 1 cm. The phallus ridge is not ectodermal, but endodermal in 

 origin. The epithelial lamellae of the phallus ridge do not diverge, 

 hence there arises neither a sex groove nor distinctly projecting sex folds. 

 The sinus urogenitalis never opens out into a sex groove. The male 

 sexual organ does not arise by longitudinal growth of the genital pro- 

 tuberance, but the small phallus is displaced navel-wards, and transformed 

 into glans and plandarium. The urethra arises by energetic growth of 

 the endodermal basis, independently of the raphe. The female phallus 



* Arch. Mikr. Anat., lxvii. (1905) pp. 1-17 (1 pi.), 

 t Anat. Anzeig., xxvii. (1905) pp. 511-20 (8 figa.). 

 % Morphol. Jahrb., xxxiv. (1905) pp. 248-320 (2 pis. and 60 figs.). 



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