28 SUMMARY OF CURRENT RESEARCHES RELATING TO 



time, the surrounding connective tissue (skeletogenous layer) has formed 

 a homogeneous sheath — the cuticula 6celeti or skeletogenous sheath. Tho 

 fibrous sheath is thickened intervertebrally, where the notochordal epi- 

 thelium is also better developed than round the future vertebra. When 

 these sheaths are fully differentiated round the notochord, a mass of cells 

 appears between the notochord and the medullary canal, in the regions 

 which form the future vertebrae. These cells spread above the nerve- 

 cord, become cartilaginous, and form the superior arches of the verte- 

 bras ; together with the skeletogenous sheath they form bone. At the 

 same time the perichordal tissue forms intervertebral swellings, whoso 

 cells rupture the skeletogenous sheath in three places, and come to lie 

 between it and the underlying elastic sheath. They there form the 

 intervertebral cartilage, and, as Gadow first showed, are the homologues 

 of the interdorsalia and interventralia of other Anamnia. The cells of 

 the intervertebral cartilage spread backwards and forwards from their 

 points of entrance, splitting the skeletogenous sheath from the elastica 

 externa, and increasing more and more in number, and finally by division 

 into two halves, form the articular surfaces between the contiguous 

 vertebras. At the time when this occurs, the cells of the notochordal 

 epithelium in the region of the vertebrae proliferate and form the plugs 

 of cartilage within the vertebrae, which have thus no connection with 

 the intervertebral cartilage, but originate from notochordal cells. 



Dorsal Groove and Dorsal Suture in Gastrula of Triton.* — Prof. 

 Hermann Braus finds that his statement that in the embryo of Triton 

 alpe8tris a primitive or dorsal groove and a dorsal suture (blastopore 

 suture of Semon), occur on the future dorsal region, has been disputed 

 by Rothig, who investigated T. tseniatus. He therefore figures the ap- 

 pearances observed in both living and preserved eggs of T. alpestris ; in 

 both cases the structures mentioned can be clearly seen during certain 

 stages. 



Larva of Plethodon cinereus. f — Dr. T. H. Montgomery, jun., de- 

 scribes the terrestrial larvae of this American Urodele. He found five 

 eggs beneath a stone with the female curled round them. The eggs 

 were of relatively large size, and contained larvae with threo pairs of 

 gills, and the limbs fully formed. The posterior limbs were larger than 

 the anterior. The eggs showed a large yolk-mass, round which the 

 larvae were curled, but close examination showed that this is not a yolk- 

 sac, but an integral part of the intestine. The anterior and posterior 

 regions of the intestine are both tubular, but the middle region is made 

 up of large yolk-cells. 



Development of Frog's Retina.:}: — Dr. Luigi Barbadoro finds that 

 all the layers of the retina increase in size in Rana esculenta during the 

 passage from the larval to the adult state. He gives a table showing 

 the thickness of the whole retina, and of each of its constituent layers, 

 in various stages of development from specimens of 5 mm. in length, to 

 adults of 65 mm. The cells of all the strata multiply, and their nuclei 

 increase in number, and change in form and size. 



* Anat. Anzeig., xx. (1901) pp. 238-40 (5 figs.). 



+ Proc. Acad. Nat. Sci. Philadelphia, 1901. pp. 503-8 (1 pi.). 



X Anut. Anzeig., xix. (1901) pp. 597-601 (3 figs.). 



