PSYCHE. 



25 



from each other ;ind each still abuts 

 a supra-oesophageal ganglion. As to 

 the median ectodermic thickening in 

 Fig. 6 (VD) which is destined to form 



fVlus 



Fig. 7. SagUt.it section of anterior end of embryo just 

 before involution of head; M, mouth; /.intestine; I'D, 

 ventral disc ; DD, dorsal discs ; Miis, muscle projection ; 

 /'. ventral side ; Z>, dorsal side. 



tlie ventral cephalic disc, it migrates for- 

 ward with the stomatodeum, sutieringat 

 the same time a slight invagination, 

 and finally takes a position immediate- 

 ly below the month (V D, Fig. 7). 

 The relation of the other organs of 

 the anterioi' portion of the embryo to 

 the discs, is well shown in Fig. 7. 

 Just dorsad of the mouth and between 

 it and the metlian portion of the dorsal 

 discs is seen a muscukir projection 

 (Mus.) whose later history will be found 

 to be interesting. 



A verj' important change now takes 

 place in the development of the animal, 

 namely, the involution of the embry- 

 onic head. An ectodermic fold starts 

 just posteriad of the discs, both dorsal 

 and ventral, and grows rapidly forward 

 towards and over the mouth. The 

 inouth, with the ventral disc just ventrad 

 of it and the muscular projection and 

 dorsal discs just dorsad of it, is rolled in 



by this process. A new mouth is formed 

 (M, Fig. S) and just back of it 

 is a new portion of the intestinal tract 

 (P, Fig. S). This is the so-called phar- 

 ynx of Weismann and Van Rees, de- 

 scribed by them in the larva of Musca. 

 Just back of the pharynx is the oesopha- 

 gus and the old mouth, dorsad of which 

 is the muscular projection (Mus. Fig. 



Fig- S. Sagittal section of embryo just after iiivoiutioii of 

 head; M, mouth; P. pharynx; DD, dorsal discs; \'D, 

 ventral disc ; /, intestine ; Mus, muscle projection ; ^ ', ven- 

 tral side ; D, dorsal side. 



S). This projection becomes in the 

 larva the most active organ in the ani- 

 mal. It is a sucking tongue and by its 

 regular pulse-like contractions causes a 

 flow of the milk-like secretion, which 

 is present in the uterus and forms 

 the food of the lar\a, into the mouth.* 

 Connected with the dorsal wall of the 

 pharynx is the fused median portion 

 of the dorsal discs (DD, Fig. S) whicli 

 in Fig. 7 opened to the outside of the 

 embryo. The distal portions of the discs 

 represent diverticida which extend back 

 to the supra-oesophageal ganglia. On 

 the ventral side of the pharynx is tb.e 

 ventral disc (VD, Fig. S) whicli show.s 

 a slight invagination and which is des- 

 tined to undergo considerable changes 



