236 THE EMIUiYOLOOV OF THE HLOIV-FLY IN THE EGU. 



modified it, regarding the parablast ceils as originating from 

 the edges of the newly-formed blastoderm. 



CJuite recently Wielowiejski [121], so far as insects are con- 

 cerned, has returned to the original view of His. He says : ' I 

 have traced the parablast cells directly to the segmentation 

 spheres of the yelk.' Most embryologists trace the para- 

 blast to the blastoderm itself, and in cases where there is no 

 large food-yelk it is difficult to see what other origin it can 

 have ; but where a large food-yelk is present, it seems to 

 me that its origin from the yelk is probable, and in insects 

 I believe it is formed from the centrolecithal segmentation 

 cells. 



Although Graber and others trace the development of the fat 

 bodies and other connective tissues to the epiblast, it appears to 

 me that the evidence in favour of this view is such that it is 

 equally favourable to that which I have adopted. Moreover, 

 there is strong reason to believe that the mesenchymatous 

 tissues of the imago are not developed from the imaginal discs, 

 but originate from the immigrant blood cells of the larva and 

 the stellate cells of the trachea;, as Viallanes and Kiinckel 

 d'Herculais maintain ; whilst the muscles, which represent the 

 true mesoblast, originate directly from the disc epiblast, as 

 Ganin insists. If this is so, then analogy is in favour of the 

 origin of the connective tissues of the embryo from the un- 

 differentiated yelk-cells, rather than from the specialised 

 epithelium of the blastoderm. 



The Relation of the Germ to the Vitellus. — The ova of the meta- 

 zoa either consist of a germ-yelk which undergoes equal 

 segmentation, holoblcistic ova ; of a germ which is loaded by 

 food-yelk in which segmentation is unequal; orof a germ which 

 undergoes complete segmentation, and a yelk which either does 

 not segment at all or in which the segmentation, if it can be 

 called segmentation, takes place at a later period when the 

 morula produced by the segmenting germ is already converted 

 into a blastoderm, entirely or partially, surrounding the yelk ; 

 which is subsequently absorbed in the nutrition of the 

 embrvo. 



