708 SUMMARY OF CURRENT RESEARCHES RELATING TO 



wall, that it is paired and solid to start with, that the mesodermic en- 

 velope takes an active share in the growth, but that this growth is largely 

 due to the division of the primitive cells forming the primordium. 



Comparative Embryology of the Swim-Bladder.* — Fanny Moser 

 points out (1) that in some fishes, e.g. Rhodeus and carp, the swim- 

 bladder is constricted like an hour-glass, lies in the middle line dorsal 

 to the gut, with a narrow pneumatic duct, which opens into the caudal 

 region of the bladder ; (2) that in another set, e.g. trout and salmon, 

 the swim-bladder is a long narrow sack, somewhat to the left of the gut, 

 with a wide opening entering the latter slightly to the left ; and (3) that 

 in others, such as stickleback, the swim-bladder is a wide sack, dorsal to 

 the gut, and without pneumatic duct. 



In groups (1) and (3), as von Baer and others have pointed out, the 

 swim-bladder arises somewhat to the right of the gut ; in group (2) the 

 origin is quite dorsal, but not quite median, being again to the right. 

 The author briefly discusses the various re-arrangements in the course 

 of growth. His point is, that there is some measure of plasticity in the 

 mutual relations of gut and swim-bladder, and that this takes the edge 

 off the contrast often drawn between the dorsal origin of the swim- 

 bladder and the ventral origin of the lungs. It will be recalled that 

 the dorsal lung of Ceratoclus arises from a ventral primordium. 



Early Development of Lepidosteus.f— A. C. Eycleshymer describes 

 the cleavage of the ovum of Lqiidosteus, which some have described as 

 meroblastic, others as holoblastic. It may possibly be of a heterogeneous 

 character, but the author regards the occurrence of complete holoblastic 

 cleavage as very improbable. A comparative study of the cleavage 

 among the Ganoids reveals in this small group of fishes a most interest- 

 ing series of transitional forms. Beginning with the holoblastic egg of 

 Acipenser, which closely resembles the Amphibian egg, we pass to the 

 modified holoblastic as exhibited by the egg of Arnia, thence to the egg 

 of Lepidosteus with its meroblastic tendencies, and from this the typical 

 meroblastic condition of the Teleost is readily derived. 



Evolution of Vertebrate Limbs.J — P. le Damany discusses the 

 familiar fact that while homologous limbs are often different in position 

 in relation to the body, they are isotropic throughout. A young croco- 

 dile in the egg has its limbs orientated almost in the same way as in 

 Mammals, but after the animal has begun to crawl about the original 

 orientation disappears. The author's point is, that the embryonic 

 orientation is adaptive to the conditions of growth within the narrow 

 compass of the egg. In another paper § the author maintains that the 

 torsion of bones, familiar in the ribs, is due to muscular action in the 

 course of development. 



b. Histology. 



Canaliculi in Ganglion Cells. ||— Rachel Pewsner Neufeld discusses 

 the " Saftkanalchen " in the ganglion cells of the spinal cord of the 



* Anat. Anzeig., xxiii. (1903) pp. 609-11. 



t Decennial Publications, Univ. Chicago, x. (1903) p. 17 (2 pis.). 



j Trav. Sci. Univ. Rennes, i. Fasc. iii. (1902) pp. 333-8. 



§ Tom. cit., pp. 371-4. 



|| Anat. Anzeig., xxiii. (1903) pp. 424-4G (2 pis. and 1 fig.). 



