242 Robert AVilhelm Ilejnicr. 



&^ 



probably, because of their small size, and similar appearance to 

 other embryonic cells. In a 1 cm. larva he described as rudimentary 

 germ-glands ''two pairs of encapsulated groups of small embryonic 

 cells, situated in the fifth abdcmiinal segment embedded in the fat- 

 bodies on either side of, and dorsal to, the alimentary canal." The 

 pole-cells were considered by Lowne as ''the segmentation spheres 

 of the vegetative pole of the egg." 



Escherich (1900) began his study of the embryonic development 

 of Musca vomitoria and Lucilia cccsar after the formation of the 

 blastoderm. He traced the history of the pole-cells from this time, 

 until the germ layers were completely separated. The pole-cells 

 were first discovered as a group lying in a posterior enlargement of 

 the ventral groove. From here they were carried around upon the 

 dorsal side by the growth of the "Mittelplatte." The pole-cells 

 migrated from here into the embryo through a "Pokellencanal ;" 

 they were last observed in the lumen of the '^'hinterer Entodermkeim" 

 which Avas connected with the inner end of the proctodeal cavity. 



Lecaillon (1900) has made the following general statement con- 

 cerning the origin of the germ-cells in Culex pipiens. "L'etude 

 du developpement embryonnaire des insectes montre que, chez ces 

 animaux, les cellules sexuelles, les (jonadcs, se sei)arent de tres bonne 

 lieure des cellules somatiques et seuddent ctre toujours de nature 

 ectodermique. Elles se groupent bientot en deux petites masses 

 pleines s'entourant chacune d'une enveloppe fonnee de cellules meso- 

 dermique aplaties. Souvent ces deux petites masses ne se modifient 

 plus, an moins dans leur structure intime, et restent telles quelles 

 jusqu' a la fin du developpement embryonnaire; ce sont les rudiments 

 ou ebauches des organes genitaux. . . Pendant toute la duree 

 de la vie larvaire, les organes genitaux demeurent a I'etat d'ebauche. 

 Celle-ci consiste en deux petits massifs cellulaires situes dans la 

 sixicme anneau abdominal." 



The most recent and best contriljution to the genesis of the pole- 

 cells of Diptera is that of Woack (1901). He found a dark gran- 

 idated layer ''Dotterplatte" at the posterior end of the q^^ of Cal- 

 Uphora erylhroccphala similar to that discovered by Ritter (1890) 

 in Cliironomns. Each pole-cell took part of this layer of granules 



