424 A Three Weeks' Human Embryo 



material has been accumulating in the form of specimens, models, draw- 

 ings, and notes with preliminary papers,'" "* until it seems clear that the 

 statements presented with regard to the central nervous system of this 

 embryo are not artifacts due to shrinkage, nor abnormalities, but that the 

 individual characteristics of this specimen may be depended upon to 

 represent one phase of development. This phase is probably transitory 

 because nothing exactly like the neuropore in this specimen has been 

 found in any other specimen examined, though stages both older and 

 younger are seen. On account of the clear demonstration of important 

 facts in transitional stages it seems worth while to record them fully. 



TTie cephalic end of the hrain tube and its relation to a serial order of 

 parts. — von Baer ^ in 1828 represented as perfectly obvious the original 

 cephalic end of the body, including the neural plate, at the point where 

 the hypophysis arises. By more refined methods, Keibel ^ in 1889 showed 

 with apparent conclusiveness, that in the rabbit the neural plate, the 

 enteron, and the notochord end in an undifferentiated mass of tissue 

 which is the true cephalic end of the body, and corresponds with the point 

 indicated by von Baer. 



From another view point, the place of final closure of the neuropore has 

 been considered by some a crucial test for determining the end of the 

 brain tube. His ^^ found it at the optic recess and found also that just 

 before final closing, the neuropore is a slit, includes the recessus infundi- 

 buli, chiasma, recessus opticus and the olfactory lobes, and extends to the 

 dorsal end of the lamina terminalis. That is, in fact. His really agrees 

 with von Baer and Keibel as to the original condition of the cleft between 

 the neural plates. 



Kupffer," in sharks and some other fishes, found that the final closure 

 of the neural tube occurs between the olfactory lobes at a point which he 

 calls the lohus olfactorius impar. Herrick ^ in discussing these widely 

 divergent views concluded that the final closure had no necessary relation 

 to the morphologic front of the brain but that the recessus infundibuli is 

 the primitive cephalic end. Herrick, therefore, also goes back to von 

 Baer's original location of this point. 



Accepting then the von Baer-Iveibel observations as correct, I 



^ von Baer, K. E., Ueber Entwicklungsgeschichte der Thiere. Konigsberg, 

 1828. 



=^ Keibel, F., Arch. f. Anat. u. Phys. Abth., 1889. 



=^His, W., Arch. f. Anat. u. Phys., Anat. Abth., 1892. 



" Kupffer, C, Studien zur vergleichenden Entwicklungsgeschichte des Kop- 

 fes der Kranioten. Heft 1. Miinchen u. Leipzig, 1893. 



=^ Herrick, C. L., Jour. Comp. Neur., Ill, 1893, p. XVI. 



