DEVELOPMENT IN PLANARIANS. 2JJ 



between the intestinal branches last mentioned. In some speci- 

 mens a new axial intestine had thus been formed between the 

 anlage of the head and that of the pharynx (Fig. 1 1 , B\ The 

 pharynx is beginning to be differentiated. The head is much 

 advanced in development but the chief commissure of the brain 

 is not completely formed. 



One hundred and thirty-eight hours. The axial intestine and 

 pharynx are well formed. The chief commissure of the brain is 

 well formed and the eyes have appeared. 



Subsequently the axial intestine formed between the two 

 original posterior rami becomes greatly developed at the expense 

 of the latter. 



Sometimes the process is more simple than that described 

 above, the anterior portion of one or the other of the posterior rami 

 becoming converted directly into an axial gut ; or fusion of two 

 rami may be more complete, as in the instance described in my 

 previous paper. 1 Miss Stevens (pp. cit.~] states that the accumula- 

 tion of cells for the pharyngeal anlage may begin in P. higubris be- 

 fore intestinal anastomosis takes place. We must admit, therefore, 

 that in instances of this kind the pharyngeal anlage is not formed 

 in response to stimuli arising from a definite axial intestinal cavity, 

 as seems to be the case in the embryo and in pieces cut anterior 

 to the pharynx, but rather upon the relations borne by a given 

 area of the worm to the intestinal system as a whole. 



In sexually mature worms cut immediately anterior to the gen- 

 ital apparatus the pharynx is formed much further anterior than 

 in sexually immature specimens. The reproductive organs un- 

 dergo retrograde metamorphosis. Destruction of the genitals 

 begins at the periphery of these organs where the intestines bor- 

 der upon them and extends towards the center (see Fig. 1 1, 7). 

 The process is a complex one which I shall not attempt to de- 

 scribe in this place. The ducts and glands, including the testicles, 

 also become disorganized, the last to disappear being those near 

 the posterior end of the animal. As a result of the breaking 

 down of these organs the neighboring tissues of the planarian 



1 " On the Physiology of Regeneration in Planaria macttlata with Especial Refer- 

 ence to the Phenomena of Regeneration," American Journal of Physiology, Vol. V., 

 p. I, 1901. 



