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 branch represents 
the whole of the ventral cephalothoracice 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 antenne, these are, as | 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 8, 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 thorax. 
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 (Keimwiilste of the German embryogenists). 
Their formation is one of the earliest phenomena in the evolution 
of the Aphides; for it takes place part passu with the formation 
of the primitive streak, and consequently long before the ap- 
pearance of the zomites 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 summary. 
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- 
