266 CHARLES RUSSELL BARDEEN. 



distinctly developed. In D the assumption of adult condition is 

 much more advanced. The base of the pharynx now extends 

 nearly perpendicular from the dorsal to the ventral body wall. 

 Head and tail regions are well developed. 



The first indications of a definite nervous system appear in the 

 region of the head at about the stage of development shown in 

 Fig. I, C. The outlines of the nervous system are, however, at 

 this period too indefinite for satisfactory illustration. There are 

 apparently bilateral anlages soon united by a commissure. The 

 lateral cords gradually extend from the region of the brain 

 towards the tail. At the period shown in Fig. I, D, the central 

 nervous system has the form there shown somewhat diagram- 

 matically. 



The median nerves and commissures and the lateral nerves 

 probably arise by outgrowth from the central nervous system. 

 Nothing is known of the development of the peripheral plexus. 



Reproductive organs are developed only after the worm has 

 advanced far in development. Curtis, (pp. <://.) has followed the 

 development of the reproductive organs in Planaria viacitlata. I 

 shall therefore attempt no description of the process in this place. 



HlSTOGENESIS IN THE EMBRYO. 



The development of the various tissues during the formation 

 of the structures characteristic of the adult is difficult to follow. 

 For the sake of making a comparative study of the size and 

 structure of the various cells at different stages of development 

 I have used a uniform method of technique. The organisms 

 were first anaesthetized with chloretone ' to cause relaxation and 

 to prevent the pre-mortem contraction which killing fluids are 

 apt to stimulate. They were then fixed in a one- half saturated 

 solution of corrosive sublimate to which acetic acid was added, 

 carried through alcohol solutions of increasing strengths up to 

 absolute alcohol, and thence into xylol. The tissues were 

 embedded in paraffine, cut into sections 3^3 to 6^3// thick, 

 and stained in dilute Delafield's haematoxylin, followed by a 

 solution of Congo red in water. A little Congo red was also 

 added to the alcohol used in dehydration. The figures have 

 been drawn to a fixed scale by means of the camera lucida. 



1 A term applied commercially to chloroform-acetone. 



