THE FORMATION OF THE GERM-LAYERS. 



315 



satisfactorily explained by interpreting the parts differently, as Graber for- 

 merly did (No. 27). We may perhaps assume that, in the Muscidae, as in 

 Chironomus, the posterior end of the germ-hand not only sinks into the yolk, 

 but also makes a hook-like bend inwards, so that the germ-band in transverse 

 sections of this region is cut through three times. In this way, the posterior 

 end of the germ-band, sunk into the yolk, and the part lying in reality anterior 

 to it, but in transverse section appearing on the dorsal side of the egg, are, 

 by means of the still open gastrula-groove, in communication in such a way 

 that, in a series of transverse sections, the lumina of portions of the gastrula- 

 furrow belonging to these two parts flow together, thus yielding the peculiar 

 dumb-bell-shaped figure. By this assumption, the invagination which 

 Yoeltzkow and Graber (No. 28) erroneously held to be the proctodaeum 

 would more correctly appear as the so-called germ-prominence (p. 276), and 

 the lumen of this invagination would then 

 have to be considered as the amniotic 

 cavity, and the aperture at its dorsal side, 

 not as the anus, but as the aperture of 

 that cavity. The proctodaeum seems to 

 appear only later in the form of an invagi- 

 nation from this cavity. This view is 

 supported throughout by Ritter's obser- 

 vations of the development of the procto- 

 daeum (No. 71). 



We must here mention Grader's view 

 of the presence of a lateral gastrulation in 

 the Muscidae. Graber finds, in the germ- 

 band of the Muscidae, near the median or 

 principal gastrula - furrow, lateral folds 

 which are specially marked in the most 

 anterior and posterior parts of the germ- 

 band, and which are said to give off 

 elements to the lower layer. These paired 

 folds, which were already known to 

 BtJTSCHLi (No. 12) and Yoeltzkow (No. 

 85), and which mark the lateral edges of 

 the germ-band, are, according to Graber, 

 supplementary gastrula - furrows which 

 serve the purpose of supporting the gas- 

 trula-furrow in its plastic activity in the 

 formation of the lower layer. Graber has, 

 however, not proved that elements are 

 given off from these lateral folds to the 

 lower layer. Since it was already known 

 to Voeltzkow that, in the stage under 

 consideration, the portion of the blasto- 

 derm not taking part in the formation of 

 the germ-band shows a great tendency to 

 the formation of folds, these folds probably 

 come under this category, and ought not 

 to be regarded as connected with the 

 further development of the embryo. 



—Ae 



Fio. 156. — Flask-shape gastrula-stage 



of Chnlieodoma (after Carri£re). 

 /, folds which bound the middle 

 plate laterally (lips of the blasto- 

 pore) ; m, the partly segmented 

 middle plate (here mesoderm-rudi- 

 ment) ; s, the segmented lateral 

 plates (later ectoderm of the germ- 

 band); ve, anterior entoderm -rudi- 

 ment ; he, posterior entoderm -rudi- 

 ment. 



