148 



THE HUMAN EMBRYO. 



Human Embryos of the Fourth Week to the Fourth Month. 



The following series of illustrations (Figs. 96-113 inclusive) are from specimens 

 in the Harvard Embryological Collection, all normal or nearly so. To facilitate 

 comparison figures 97-107 are uniformly magnified five diameters, while figures 108-113 

 are life size. 



Embryos of Four Weeks, 7.5 to 8.0 mm. They are characterized especially 

 by the extreme development of the neck-bend. The fourth and fifth branchial 

 arches are entirely buried in the cervical sinus, and the third arch is turning 

 in. In other words, the process of invagination of the sinus, though far advanced, 



FIG. 96. HUMAN OVUM WITH EMBRYO OF 9.4 MM. 

 THE CHORION HAS BEEN PARTLY REMOVED TO 

 SHOW THE EMBRYO. X 3 diams. (Minot Collec- 

 tion, 275.) 



FIG. 97. HUMAN EMBRYO OF 9.6 MM. 

 SERIES 1001. X 5 diams. 



is not completed. The invagination of the ectoderm to form the lens is still open, 

 though about to close. The back of the embryo is partly straightened. The limb 

 buds are beginning to expand at their distal ends to make the anlages of the hands 

 and feet. 



Embryos of Twenty-eight to Thirty Days, 8.0 to 10.0 mm. The form of hu- 

 man embryos at the end of the first month is very variable, and it has not been 

 possible hitherto to establish with certainty a typical normal shape. Their length 

 varies because the head begins to rise with accompanying diminution of the neck- 

 bend, hence the length may be increased by a change of form without a correspond- 

 ing growth of the embryo as a whole or advance in structure. Figure 96 illustrates 

 the proportions of the embryo, yolk-sac, and chorion at this stage. Figure 97 shows 

 an embryo of 9.6 mm. with the yolk-sac and stalk. In this specimen the oblitera- 



