OF THE BRAIN OF THE HUMAN EMBRYO. 27 



branches of the aorta, it is apparently possible to obtain a spontaneous migration 

 without the aid of collateral loops. Here the result is obtained by unequal growth 

 of the endothehal walls. As a subhead under spontaneous migration we might 

 include replacement channels. In this process there is the formation of a new channel 

 and the obliteration of an old one. A replacement channel differs from other 

 spontaneously migrating channels in that it is not a gradual and progressive change 

 in position, but an abrupt and immediately complete one. Furthermore, the new 

 channel lacks the morphological characteristics of the old one. An illustration of a 

 replacement channel is the channel dorsal to the otic capsule (transverse sinus), 

 which supplants the otic portion of the primary head- vein. 



The lateral telencephahc veins of Markowski apparently correspond to the 

 inferior cerebral veins of the adult, so we will label them in that way. Though 

 emptying into the dural system, they develop their course through the intradural 

 membranes and become typical cerebral veins. It is interesting to note that in the 

 21 mm. embryo certain definite topographical points in the transverse sinus are 

 already determined, namely, the jugular foramen, the location of the endolymphatic 

 sac, the points of entry of the superior petrosal sinus and of the inferior cerebral 

 veins. Thus we see that more than half of the sinus is already established and 

 that it is the terminal or jugular portion that is established first. The remainder 

 of the sinus is relatively late in assuming a permanent form, which is doubtless 

 the result of the prolonged period of growth of the cerebrum, making a continued 

 adjustment of the tentorial plexus necessary. Even in embryos 50 mm. long, 

 which we are about to examine, the proximal end of this sinus is still in the forma- 

 tive stage. Before leaving this stage, and once more comparing figures 3, 4, and 

 26, it should be pointed out that the great drainage-channels of the head are effi- 

 ciently adapted to the drainage of its parts as then existing. We are not to think 

 of them as busily engaged in building the transverse and sagittal sinuses, but as 

 carrying on their functional activity in the best manner possible for the moment 

 and with regard to the available space and the amount of given work. The com- 

 pleted transverse and sagittal sinuses will come in good time, as determined by 

 later conditions. 



To cover the period of embryos about 50 mm. long, the writer examined four 

 series belonging to the Carnegie Collection: No. 886, 42 mm. coronal; No. 84, 50 

 mm. transverse; No. 96, 50 mm. sagittal; and No. 448, 52 mm. sagittal, injected. 

 There was also an embryo of about the same age (No. 458, 54 mm.) that had been 

 injected with India ink, the head of which was removed and partly dissected, and then 

 cleared after the Spalteholz method. This gave excellent total views of the blood- 

 vessels. The profile reconstruction shown in figure 6 is based on series No. 96 and 

 was made by preparing tracings on transparent paper which were then superimposed 

 and a composite tracing made of the whole series. This is about the same stage 

 that is shown by Markowski in his figure 4. The reconstruction shown in figure 27 

 is of a younger embryo and was made after the Born-Lewis method. 



At this period the arterial supply and the venous drainage of the head are 

 established along channels that correspond fairly well to those found in the adult. 



