CHAP. XLI.] OF THE HUMAN EMBRYO. 591 



reptile or a fish. All seem to be developed upon one general 

 plan. The organs are evolved in the same order, and the organic 

 functions in the manner of their performance are precisely analo- 

 gous. After this point has been reached, however, the distinctive 

 characters are well marked, and then it is very easy to say 

 whether the embryo is to assume permanently the condition of 

 a fish, reptile, bird, or mammal. 



One of the youngest human ova which has been examined, is 

 described by Mr. Wharton Jones, and was aborted in the third or 

 fourth week after impregnation. It was about the size of a pea, 

 and was probably detached at a somewhat earlier period. Villi 

 were seen on one side only of the chorion. At one end of the 

 ovum, lying in the midst of a gelatinous material, was the germi- 

 nal vesicle, but the embryo was not yet visible upon it. 



Dr. Allen Thomson has examined two human ova at a some- 

 what later period. One was a quarter of an inch, and the other 

 half an inch, in diameter. The embryo was about one line in 

 length (Fig. 282). It lay nearly flat upon the yolk-sac, no con- 

 striction being as yet formed. Fig. 232. 

 The dorsal laminae were dis- 

 tinct, and had not yet united 

 together. There was neither 

 allantois nor amnion. This 

 ovum was examined about 

 fifteen days after conception. 



MM. Pockels and Coste 

 have had an opportunity of 

 examining an ovum about 



this period, in which the A Human ovum described and figured by Dr. Allen 



i-iw^l-v'1'rt 1 /-. "1 -r>A 11 Thomson, about fifteen days after conception, magnified 



Umbilical Vesicle and allan- ten diameters, a. The open vertebral canal, bounded by the 



f r\\* WOT./! oooYi TrovTT rliof innfUr lami se dorsales. 6. Foldsof the intestinal groove, c. Po- 



tois were seen very distinctly. sition of the heart ^ Yolk sac . e . A piece of membrane 



The CmbrVO lav in the perhaps connected with tne formation of the Chorion. 



amniotic fluid ; but the allantois had not yet become connected with 

 the inner surface of the chorion. In another embryo described by 

 Dr. Allen Thomson, about five or six weeks after conception, the 

 allantois was attached to the chorion, and there existed an umbi- 

 lical chord, which was not yet enclosed in its sheath formed by the 

 amnion. It was the eighth of an inch in length. The heart pro- 

 jected in the form of a looped vessel from the anterior aspect of the 

 body. The intestine was straight and opened into the yolk-sac, and 

 the opening of the anus was not yet formed. The umbilical 

 vesicle was becoming narrower near the embryo. Two branchial 



