No. 3.] A HUMAN EMBRYO TWENTY-SIX DAYS OLD. 46 1 



on the left side ; it lies deep in the sinus praecervicalis, and 

 is almost covered by the third arch. 



The clefts are irregular in shape, as shown in the figure ; 

 and the first, second, and third show marked depressions at 

 their dorsal ends, which indicate the blending of the ectoderm 

 with the seventh, ninth, and tenth nerves. 



On the dorsal side of the branchial region, alike on both sides 

 of the head, there is a marked depression which lies immedi- 

 ately over the otic vesicle. The vesicle, however, is fully sepa- 

 rated from the ectoderm. 



The protovertebrae are more marked on the right than the 

 left side ; twenty-seven on the right, and twenty-four on the 

 left. On the right only the last seven cervical, all the dorsal 

 and lumbar, and five sacral are visible ; while on the left two 

 occipital, all the cervical and dorsal, and but two lumbar are 

 seen. The sections, however, show that the muscle plates are 

 the same on both sides. 



The extremities are well marked, the anterior being some- 

 what larger than the posterior. The anterior on the right side 

 is flat, and bent directly towards the median line ; while on the 

 left it hangs away from the mouth. The posterior on the right 

 is bent towards the head, and on the left side it is simply an 

 oval mound. 



Upon the body proper there are three marked elevations : 

 two for the heart, and one for the liver. In general, these 

 elevations are the same on both sides. 



The umbilical cord is large, and lies on the left side of the 

 body, as described by Waldeyer 1 and by Janosfk. 2 In most 

 embryos described it lies on the right side. The cord is short, 

 and midway between the embryo and its attachment to the 

 chorion it shows a decided enlargement. The umbilical vesicle 

 is large, measuring 7 mm. in length and 5 in diameter. 



Method of Study. 



The embryo was stained with alum carmine, and cut into serial 

 sections 15 /j, thick. Although thinner sections undoubtedly 

 would have been better to study the histology, for my purpose I 



1 Studien des physiol. Inst, zu Breslau, Heft 3, 1865. 



2 Arch. f. mik. Anat., Bd. 30. 



