SUMMARY OF CURRENT RESEARCHES. 471 



man, rat, armadillo, opossum, guinea-pig, and bat. A comparative study 

 of Mammalian spermatogenesis reveals the absence of typical lietero- 

 ohromosomes in mongoose, cat, squirrel, rabbit, and pig. Heterochro- 

 mosomes are clearly present at synapsis and prophase in the primary 

 spermatocytes of white mouse, sheep, horse, mule, dog, and bull. 



At certain stages the heterochromosomes (chromosome nucleoli) 

 appear single (accessory chromosome or monosome) ; at other stages 

 they appear double or bipartite (as if due to the splitting of an accessory). 



The absence of discernible heterochromosomes in the cat's sperma- 

 togenesis and their conspicuous presence in the oocytes, suggests that 

 they are present in one sex or the other in all forms. This suggests 

 an enquiry into the oocytes of mongoose, squirrel, rabbit, pig, and 

 similar forms. The author inclines to the hypothesis that the presence 

 •of two ir-elements in the zygote may prevent, and the presence of one 

 may permit, the development of the male sex. 



Phylogenetic Significance of Mesoblast and Coelome.* — W. Salensky 

 has reached the following conclusions. 1. The radial disposition of the 

 primordium of the mesenchyme and its precocious appearance in animals 

 with bilateral symmetry suggest that the latter are descended from 

 radially symmetrical ancestors. 2. The most probable way in which 

 bilaterally symmetrical forms have been derived from radial forms is by 

 the transformation of radial canals into coelomic sacs. 3. In this trans- 

 formation of a radial into a bilateral type the cyclomerism of the former 

 is transformed into the metamerism of the latter. 4. This leads to 

 the view that the first descendants of radial types were metameric 

 animals like Annelids. 5. Types in which the metameric structure is 

 not distinct have arisen from metameric types by retrogression. 



Development of Derivatives of the Visceral Cleft Region.f— H. 

 Rabl has studied this in the guinea-pig. The thyroid develops from the 

 epithelium of a median groove of the buccal floor in the region of the 

 second visceral arch. Several short diverticula grow out, become bound 

 together, form a reticulate primordium, get shunted backwards, and 

 begin to differentiate. The second branchial pouch develops in a 

 caudal and ventral direction, and forms a broad transverse wing and a 

 sagittal duct springing from its caudal-lateral corner. This duct soon 

 atrophies. 



The third pouch divides into a median primordium of the epithelial 

 body and a lateral primordium of the thymus. The median part loses 

 its lumen ; it forms by backward growth the parathyroid strand and, 

 anteriorly, a shorter less constant strand. The lateral portions show 

 active proHferation, and form a sac which gives off processes. _ The 

 vesicula cervicalis arises from the superficial epithelium of the third to 

 the fifth arch and the retrobranchial ridge. 



The fourth pouch arises along with the ultimo-branchial body ; it 

 forms an epithelial body and gives off posteriorly the fourth parathyroid 



* Resumes des Communications, 9e Congres Interuat. Zool. Monaco, 1913, 

 ser. 2, pp. 12-14. 



t Arch. Mikr. Anat., Ixxxii. (1913) pp. 79-147 (5 pis. and 2 figs.). 



