14 DEVELOPMENT OF CEREBRO-SPINAL SPACES IN PIG AND IN MAN. 



nervous tissue. In general, it was found that this rule held : an arterial extravasa- 

 tion always took place from the perforating capillaries, while a similar venous rup- 

 ture occurred in the veins themselves. 



Mall made similar observations on living pig embryos from 30 to 80 mm. in 

 length, with analogous results. But when, in these embryos, the arachnoid spaces 

 were completely filled by an intraventricular injection of india ink, no passage of 

 the granular injection into the veins or sinuses occurred. The ventricular injection 

 flowed into the extraventricular spaces "through the medial opening of the fourth 

 ventricle." From the spinal cord the ink extended for a short distance along the 

 main trunks of the spinal nerves. In the larger embryos (above 50 mm.) the ink 

 usually gushed from the mouth, reaching it by way of the Eustachian tube. Using, 

 in the pig embryo, the heart as the mechanism for injecting the ink, extravasation 

 from the cerebral vessels in the arachnoid spaces occurred. 



In one human specimen of 90 mm., Mall found both the arachnoid spaces and 

 the cerebral ventricles filled with india ink after an arterial injection of that sus- 

 pension. He states: "The injection passes through the medial opening into the 

 fourth ventricle (Magendie), and apparently the ventricles are injected through this 

 opening from the arachnoid." 



To His (25 ) and to Kolliker (31 > belongs the credit of first having established on a 

 firm basis the development of all the meninges in man from mesenchyme. This 

 perimedullary layer of mesenchyme Salvi (50) called the "primitive meninx" a term 

 now used extensively in comparative anatomy. The primitive meninx divides into 

 two layers, the outer forming the dura and the inner the pia-arachnoid. Sterzi^ 531 , 

 working on the development of the human spinal meninges, advanced a view similar 

 to that of Kolliker. The perimedullary mesenchyme (the "primitive meninx") 

 divides into two portions, one hugging the inner surfaces of the vertebrae and the 

 other adhering to the cord. This inner layer of the perimedullary mesenchyme, 

 according to Sterzi, should properly be termed the "primitive meninx," as it divides 

 subsequently into dura and the "secondary meninx," which in turn forms both 

 arachnoid and pia. The denticulate ligaments develop in the "primitive meninx." 

 The dura and arachnoid in human embryos are modeled up to a certain point on the 

 cord; then, with the augmentation of the subarachnoid space, they follow the outline 

 of the vertebral canal. 



His( 25) has given information regarding the development of the meninges, with 

 particular reference to the formation of the subarachnoid space. He affirms the 

 mesenchymal origin of all of the cerebro-spinal membranes. His describes the first 

 differentiation of mesenchyme to form the meninges as consisting of two zones of 

 condensation, the outer being closely associated with the developing perichondrium 

 of the vertebral column and the inner facing upon the cord. Between these two 

 zones of condensation the subarachnoid space develops, posterior and anterior 

 spaces first appearing, with later fusion laterally. These appearances were met 

 with in chicks of 10 to 12 days' incubation. Quite soon after this process of space- 

 development a separation occurs which gives rise to a complete subarachnoid space. 

 Later the splitting-off of dura from the vertebral periosteum takes place. 



