66 M. Balbiani on the Reproduction 



branch. This curved lamina, produced in this way by a bud- 

 ding of the blastoderm in the interior of its own cavity, is nothing 

 but the embryonic rudiment or primitive streak, less the ante- 

 rior part of the head; in fact the ascending brahch represents 

 the whole of the ventral cephalothoracic wall, destined to bear 

 the buccal and locomotive appendages, and the descending 

 branch the ventral wall of the abdomen. As to the elements des- 

 tined to form the anterior wall of the head with its appendages, 

 or the antennse, these are, as I have just stated, the only ones 

 which result from a transformation of the blastoderm. For this 

 purpose the latter becomes thickened in the region corresponding 

 with that against which the ascending or cephalothoracic branch 

 of the primitive streak is applied in such a manner as to sur- 

 round like a hood the base of this branch, with which this thick- 

 ened part is continued through the orifice of the posterior pole. 

 In all the rest of its extent the blastoderm becomes trans- 

 formed into a delicate membrane, which envelopes the embryo 

 as it were in a sort of sac, which isolates it from the ovarian 

 chamber. 



At this period of its evolution the embryonal streak therefore 

 presents in its totality the form of an S, of which the inferior 

 curvature represents the cephalic hood, the superior curvature 

 the rudiment of the abdomen, and the intermediate branch the 

 combined rudiments of the head and thoi'ax. 



The primitive streak divides into two longitudinal halves by 

 the formation of a furrow upon each of its faces. These two sym- 

 metrical halves, which represent the axes of the two halves of the 

 body, and betray the bilateral type of the animal, are the germi- 

 native tuberosities {Keimwulste of the German embryogenists). 

 Their formation is one of the earliest phenomena in the evolution 

 of the Aphides ; for it takes place pari passu with the formation 

 of the primitive streak, and consequently long before the ap- 

 pearance of the zonites and their appendages. As to the other 

 principal embryogenic phenomena, such as the formation of a 

 reflected superficial lamella (the Faltenblatt of Weismann), that 

 of the primitive parts of the head, the division of the germinative 

 tuberosities into transverse segments or zonites, the appearance 

 of the cephalic and thoracic appendages, &c., I can only men- 

 tion them here, leaving all that relates to these different points 

 of embryonic evolution to the memoir in which I propose to treat 

 in extenso the subject of which this note gives a summaiy. 



In proportion as the primitive streak penetrates into the in- 

 terior of the ovum, the sexual masses follow it in its movement, 

 and place themselves against the inner face of the upper reflected 

 or abdominal portion of this streak. At this period nothing yet 

 exists resembling a visceral cavity^ the streak, as in all the Arti- 



