INJECTIONS AND REPLACEMENTS IN THE CEREBRO-SPINAL SYSTEM. 25 



The two lateral extensions from the ventral sheet of the injection granules approach 

 on either side this mesencephalic eminence. The peculiar appearance of the injec- 

 tion spread caused by the chorioidal imagination of the roof of the fourth ventricle 

 is also here illustrated. 



In this specimen, then, of a pig embryo 21 mm. the periaxial spread is almost 

 complete, the only areas not entirely surrounded being the anterior mesencephalon 

 and the cerebral hemispheres. In an embryo but a few millimeters larger this 

 periaxial extension of the solution is complete. The mesencephalon first becomes 

 entirely covered by the prussian-blue precipitate, with later extension over the 

 hemispheres. This complete periaxial injection occurs usually in replacements in 

 embryos varying in length from 24 to 28 mm. 



A specimen exhibiting a complete extension of the replaced solution around the 

 central nervous system is shown in figure 8. This specimen was prepared by replac- 

 ing the cerebro-spinal fluid in a living embryo of 26 mm. and then keeping the 

 embryo alive for an hour. After fixation in an acid medium, dehydration, and clear- 

 ing, the injection was found to occupy the whole medullary-canal system and also 

 to surround completely the cerebro-spinal axis, as shown in the lateral view. The 

 striking features of this stage are similar to those observed in the younger specimens 

 the dense accumulation of granular material in the region of the roof of the fourth 

 ventricle, the surrounding of the central portion of the caudal cranial nerves, and 

 the thin pericerebral covering by the replacement mass. In addition the specimen 

 exhibits in the thoracic region an extension of the granular material laterally along 

 each spinal nerve. An observation of this peculiarity reveals the prussian-blue 

 extending outwards only as far as the ganglia on the posterior roots. 



The relationships, then, observed in an embryo pig of 26 mm. are those which 

 exist in the adult; the cerebro-spinal axis contains cerebro-spinal fluid within its 

 cerebral ventricles and within the central canal of the spinal cord, while in turn it is 

 completely surrounded by cerebro-spinal fluid within the subarachnoid space. 

 Communication between the ventricles or intra-medullary system and the peri- 

 spinal spaces occurs only in the region of the fourth ventricle. Here again the adult 

 human relationship holds. The evidence, therefore, from a study of the fluid 

 spread in a replacement experiment with the use of true solutions, indicates that in 

 pig embryos of about 26 mm. an adult distribution of cerebro-spinal fluid occurs. 



THE RESULTS OF INJECTIONS OF TRUE SOLUTIONS. 



In the preceding section there have been detailed the results of experiments on 

 living pig embryos in which the cerebro-spinal fluid of both the central canal of 

 the spinal cord and the cerebral ventricles has been replaced by a dilute solution of 

 potassium ferrocyanide and iron-ammonium citrate. After the replacement, car- 

 ried out so as to avoid any increase in the normal tension, the embryos were incu- 

 bated for varying periods of time so that the normal current of the fluid might cause 

 an extension of the foreign solution. In the experiments which will be recorded 

 in this section the same true solution was injected from an ordinary syringe and the 



