562 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Origin of Sex-cells in Rana pipiens.* — B. M. Allen finds that in 

 this frog- the sex-cells arise, in a Large proportion at least, from the 

 endoderm, from which they pass into the root of the mesentery at the 

 time when the latter is formed by the approximation of the lateral 

 plates of mesoderm. The same is true in the turtle (Chrysemys). 



Studies on Spermatogenesis.! — Jacques van Molle describes the 

 spermatogenesis of the squirrel. The fine filaments which appear at the 

 beginning of the development of the spermatocyte associate together in 

 pairs during the synapsis. The nucleoli seen in the first " leptotene " 

 stages are resolved into chromosomial filaments. The synapsis stage 

 includes three phases, and the conjugation of chromosomes occurs in the 

 middle or " amphisynaptene " phase. 



Development of Vertebrate Nerve-cord.}— Oh. van Bambeke has 

 studied this in Pelobates fuscus, where the process is peculiar. He notes 

 that, apart from Amphioxus, there are two modes of formation in 

 Vertebrates : the sides of the medullary groove may close to form a 

 medullary canal, or the axis may be a solid keel, which secondarily 

 becomes tubular. In Pelobates the medullary plate does not form the 

 walls of the medullary groove ; the closure of the groove is due to the 

 enveloping membrane (Deckschicht of Goette) ; after this is closed the 

 medullary plate gives rise to the true medullary canal. The details of 

 this peculiar mode of origin are described and figured. The author 

 points out that in Hylodes martinicensis the neural plate was found by 

 Sampson to be solid, recalling the condition in Teleosts and bony 

 Ganoids. More cases should be examined before it is affirmed that the 

 primarily solid or the primarily tubular foundation of the neural axis is 

 the more primitive condition. 



Development of the Head. — D. Filatoff§ has studied the meta- 

 merism of the head of Emys Ivtaria. 



W. Sippel discusses the structure and development of the roof of the 

 mouth in Birds and Mammals. 



A. BrachetIF concludes his study of the development of the head of 

 Amphibians, as regards sensory structures, nervous system, musculature, 

 and skeleton. 



Gill-cleft Region of Gymnophiona.** — Harry Marcus has studied 

 embryos of Hypogeophis rostratus and H. alternans. One of his general 

 conclusions is that the Gymnophiona are the most primitive living- 

 Amphibians. Some of the reasons are the following. There are seven 

 visceral clefts ; the " ultimobranchial body," developed from the last, is 

 directly homologous with that of Selachians and Geratodus ; the lung- 

 primordium may be compared w r ith the diverticulum from the eighth 

 gill-pouch of the lamprey ; a second aortic arch is formed ; the spiracle 



* Anat. Anzeig., xxxi. (1907) pp. 339-47 (5 figs.), 



t La Cellule, xxiv. (1907) pp. 259-76 (1 pi.). 



% Arch. Biol., xxiii. (1908) pp. 523-39 (1 pi.). 



§ Morphol. Jahrb., xxxvii. (1907) pp. 289-96 (3 pis. and 4 figs.). 



|| Torn, cit., pp.- 490-524 (1 pi. and 12 figs.). 



1| Arch. Biol., xxiii. (1907) pp. 193-257 (3 pis.). 



** Arch. Mikr. Anat., lxxi. (1908) pp. 695-774 (4 pis. and 12 figs.). 



