396 NOTES AND MEMOKANDA. 



the remaining parts forming merely a series of irregular masses of 

 small spheres, in which, in fact, the processes of segmentation are being 

 still carried on. 



The history of the primitive line during this period is altogether 

 different ; not only does it increase in size three or four times, but it 

 presents an altogether distinct hypoblast-layer, while the epiblast is 

 seen to be connected with cells, which will ultimately form the meso- 

 blast ; at the fourteenth hour the primitive line is simple and homo- 

 geneous, and it is with diflQculty that a faint depression can be observed ; 

 at the nineteenth hour the pit of the fourteenth has become a long 

 groove. Eunning along this line or groove there may be observed a 

 delicate filament, for which the name of epiaxial filament is now pro- 

 posed ; for this structure Dursy had proposed the name of axial, and 

 his observations are quoted and corrected in some details, the correct 

 apprehension of which the French embryologist ascribes to the use of 

 osmic acid. 



In the second period the primitive groove undergoes no changes 

 of any importance, but in the tergal zone certain modifications obtain, 

 which lead to the formation of the medullary groove and of the dorsal 

 cord ; these changes commence with the great increase in size of this 

 region ; the cells of the mesoblast extend outwards towards the sides of 

 the embryo, and also form a condensed mass at the level of the bottom 

 of the medullary groove ; this mass is the future chorda dorsalis. Much 

 of what happens in this period, as in the succeeding one, could only 

 be rendered intelligible by the aid of a number of figures. 



Of the thii'd period, the most important characters are the continua- 

 tion of the metamorphoses of the tergal zone, and the retrograde de- 

 velopment of the primitive groove ; the author refers to what he has 

 already stated as to the fourth ventricle of the brain, and the so-called 

 rhomboidal sinus, and insists on the fact that the medullary groove is 

 completely closed along its whole extent, so that the two regions just 

 mentioned owe their origin to modifications of the central canal of 

 this part. 



Dr. Duval concludes his paper with three statements ; the first, in 

 which he definitely states that the primitive groove ought to be abso- 

 lutely distinguished from the medullary groove, and that these parts 

 have different periods of, and different regions for, their development, 

 is accompanied by a detailed criticism of the works and statements of 

 earlier embryologists ; in the second, he deals with the history of the 

 three layers of the embryo, and shows that the course of development 

 of these three is different in the tergal zone to what it is in the parts 

 posterior to it ; and he brings this to bear on the statements of Eemak, 

 Waldeyer, Goette, Balfour, and Durante, who assert that the meso- 

 blast is developed from the hypoblast ; of Kolliker, who ascribes to 

 the former of these two layers a purely ectodermal origin ; and of His, 

 who would regard the epiblast and hypoblast as both being the seat 

 of origin of the mesoblast. 



The third statement is that the chorda dorsalis is solely formed 

 from the tergal zone and in front of the primitive groove, and that it 

 is formed from the hypoblast, or at any rate has the same origin as 



