1 



Mi see Uaneous. 6 5 



hatching, and I have thus been able to determine the precise signi- 

 ficance of these elements. I shall not describe the manner in which 

 they are formed in Chironomus, the facts having been described 

 in detail by MM. Robin and Weismann ; but I am not in agreement 

 with those observers as to the number of polar eel is that we meet 

 with in these insects when these bodies are definitively constituted. 

 Weismann makes them as many as twelve ; and, according to AC. 

 Robin, their number may even rise to sixteen or twenty by the suc- 

 cessive divisions of the polar cells originally formed. I have never 

 found more than eight, in the two species at least of Chirommm 

 that 1 have observed. 



The group formed by the eight polar cells is still perfectly iso- 

 lated and visible at the commencement of the formation of the blas- 

 toderm, in the space left at the posterior pole by the vitellus when 

 it has attained the maximum of its retraction. In proportion as 

 the blastoderm becomes organized, the vitellus elongates again 



towards the two extremities of the egg, and presses against the ex- 

 ternal envelope the aggregation of polar cells, which is soon com- 

 pletely concealed by the blastoderm ; but these cells do not become 

 at all confounded with those of this germinal membrane, as has been 

 supposed by the observers to whose opinion I have already referred. 

 In fact we soon see a slight impression of the blastoderm produced 

 at the posterior pole, forming, as it were, a fold of that membrane 

 towards the interior of the egg. This invagiuated part, or caudal 



extremity of the embryo, pushes before it the group of polar cells, 

 which collect into a rounded mass and always adhere loosely to each 

 other, by which means they retain their original spherical form. 



By the advance of the invagination this mass comes to be placed 

 between the caudal rudiment and the ventral surface of the egg, 

 surrounded on all sides by the granular substance of the vitellus. 

 After arriving in this position the polar cells do not again quit their 

 relations with the caudal extremity, which they follow in all its 

 positions at the different stages of developmcut. We still find them 

 there when this part has become elongated by ascending along the 

 convex or dorsal side of the egg, so as to touch with its extremity 

 the posterior margin of the head. During this ascending movement 

 the polar mass divides into two equal oval portions, placed somewhat 

 obliquely on each side of the longitudinal axis of the tail. To 

 arrive at a more complete idea of the constitution of these secondary 

 masses, it is necessary to isolate them and to submit them to the 

 action of reagents. We then ascertain that each of them is formed 

 of two spherical cells, flattened at their surface of contact. From 

 this it appears that instead of the original eight polar cells we no 

 longer find more than four, probably in consequence of a fusion, two 

 and two, of the eight preexisting cells. The reagents do not reveal 

 any enveloping membrane around each mass ; but they show that its 

 two constituent cells are in course of proliferation, by causing 

 from two to four clear nuclei to appear in the interior of each of 

 them. 



At a more advanced period of development the caudal' extremity 



Ann. d: Mag. N. Hist. Ser. 5. Vol xi. 5 



