03 G SUMMARY OF CURRENT RESEARCHES RELATING TO 



a portion of tlio primitive groove. In front of the peripheral bud in 

 the region of the embryonic shield a portion of tho primitive groove 

 emerges for a short period, in a stage corresponding to III. of the 

 above. The author emphasizes further that the region of the 

 primitive streak is free from the chorda, which afterwards comes into 

 local association with it, that the medullary groove has in its first 

 appearance no connection with tho primitive groove behind it. 



In Reptiles the primitive streak has the form of a bud or knob. 

 The primitive groove is represented by tho neurenteric canal, described 

 by Kupffer and Benecke. This canal has all tho characteristics of 

 a primitive groove: (1) it is found in the region of the bud, as in 

 Selachia and Teleostei ; (2) it is closed in front ; (3) it is utilized 

 in forming the posterior portion of the body ; (4) the chorda comes 

 into subsequent local connection with it ; (5) it comes to be a con- 

 tinuation of the neural canal ; (G) the margins of the neurenteric canal 

 (or primitive groove) represent the primitive folds, for they help, as 

 in selachians, birds, and mammals, to form the medullary groove ; 

 finally, the area of this primitive streak extends into the peripheral 

 ridge, and is there associated with sickle-like horns. The primitive 

 streak or neurenteric canal of reptiles is therefore no form of gastrula. 

 The result of Kallmann's interpretation is to maintain in the different 

 groups an intimate homology in the characters of the primitive streak 

 and in the phases which it exhibits. 



Germinal Layers of Chelonia.* — Prof. K. Mitsukuri and Mr. C. 

 Ishikawa have investigated the formation of the germinal layers in 

 Trionyx japonicus ; they find a passage which commences at the 

 blastopore and takes a forward and downward course to the ventral 

 surface, in the middle of which it opens by a circular orifice ; at the 

 dorsal lip of the blastopore the ectoblast is continuous with the 

 chorda-entoblast ; the conversion of the axial part of the entoblast 

 into chorda-entoblast proceeds from behind forwards. At the floor 

 of the blastoporic passage the ectoblast fuses with the entoblast, and 

 at this point the entoblast is given off posteriorly in all directions 

 through 180°. The line of the primitive streak is very short. The 

 med an mass formed by the fusion of the three layers appears for a 

 short space on the dorsal surface, and may be regarded as the 

 remnant of the yolk-plug of Rusconi. 



The mesoblast commences as a string of cells placed dorsally to 

 the enteric entoblast and ventrally to the ectoblast, but it is distinct 

 from both ; it only later extends into the region in front of the 

 blastoporic passage, and here it arises as a paired mass, one part of 

 which is always continuous with the chorda-entoblast, and the other 

 with the exterior entoblast. The notochord is first completed in the 

 middle, and then extends backwards and forwards. 



With regard to the formation of the blastoporic passage, the 

 authors somewhat diffidently suggest the following ; at the end of 

 segmentation the blastoderm becomes divided into two primary 

 layers ; but when the differentiation of the ectoblast has reached the 



* Quart. Journ. Micr. Sci., xxvii. (1S86) pp. 17-48 (4 pis.). 



