METHODS OF INVESTIGATION. 



17 



was impracticable on small embryos. After such a replacement the animals were 

 allowed to live for varying periods of time (up to 3 hours) and then killed. 



It was soon ascertained that the essential circulation of the cerebro-spinal fluid 

 was established in pig embryos of less than 30 mm. in crown-rump measurement. 

 Hence the ordinary method of replacement had to be discarded for some more 

 delicate system. With the realization that a simultaneous withdrawal and intro- 

 duction in a living embryo would be far preferable to a two-stage procedure, the 

 extremely simple apparatus pictured in text-figures 1 and 2 was employed. This 

 device consists of two glass tubes of uniform and like bores, suspended from above 

 by a string running over a pulley. To the tapering lower ends of these reservoirs 

 are attached rubber tubes which connect the reservoirs to two needles. These 

 needles are held at the same level by two metal brackets which can be moved at 

 will on a level glass plate. 



TEXT-FIGUKE 2. Diagrammatic representation of 

 the method of replacing the cerebro-spinal fluid 

 in a living embryo. The spinal needle is inserted 

 into the central canal of the spinal cord, while 

 the cerebral needle is introduced into one of the 

 cerebral ventricles. The canal of the spinal 

 cord and the cerebral ventricles are represented 

 by the interrupted lines. The foreign fluids 

 are introduced by the spinal needle and with- 

 drawn by the cranial. 



The apparatus is employed as follows: Both tubular reservoirs are filled up to 

 the point where the fluid is just ready to fall from the needle in a drop. This point 

 is easily obtained by filling the reservoirs slightly in excess and allowing this excess 

 fluid to run off from the needle. With the system thus in balance the needles lie 

 in the same horizontal plane and can be moved without altering the balance of the 

 solutions. The injection is made by inserting one needle into the central canal of 

 the spinal cord and the other into one of the lateral ventricles; then as the reservoir 

 connecting with the spinal needle is raised the other is lowered, so that an amount 

 of fluid equal to that introduced into the spinal canal is withdrawn from the cerebral 

 ventricles. In this way the whole contents of the cerebral ventricles and central 

 canal of the spinal cord can be slowly withdrawn without increasing the pressure 

 in the central nervous system. The initial pressure necessary to secure this flow 

 is only that required to overcome the capillary resistance of the medullary-canal 

 system. In practically all cases this can be accomplished by using a positive pres- 

 sure of less than 60 mm. of water (associated with a negative pressure of the same 

 degree) . 



