EXTERNAL FORM OF EMBRYO OF 20 MM. 171 



lately and relatively, to the body of the embryo, and has, moreover, risen so that 

 the neck bend is diminished. The limbs are beginning to show the differentia- 

 tion of the feet. Examined more carefully, the embryo offers the following 

 details : the eye, which is characteristically small, has become almond-shaped, 

 and the circular lens can be seen in the midst of it. In the embryos of rodents, 

 carnivores, and primates the eye is larger relatively than in the pig. By the 

 growth of the facial region the development of the snout has been initiated and 

 the opening of the nasal pit now appears as the external nares toward the end of 

 the short snout. The lower jaw is clearly differentiated and the slit of the mouth 

 is distinct. There has been a great growth of the regions of the fore-brain and 

 mid-brain, and it is this growth chiefly which has caused the relative expansion 

 of the head as compared with the rest of the body. The auditory groove now 

 appears distinctly as the anlage of the external meatus of the ear, behind which 

 a protuberance can be seen which is the anlage of the external concha of the ear. 

 The cervical sinus has wholly disappeared. Along the line of the back the primi- 

 tive segments are scarcely recognizable in the cervical region, but near the upper 

 limb they still show distinctly and from there are indicated with increasing clear- 

 ness as we pass toward the lower limb. The marks of the segmental divisions 

 do not extend as far on the dorsal side as in the earlier embryos, but are re- 

 stricted to what may be called the segmental ridge. Along the milk-line a series 

 of small, white, circular spots can be seen. In the specimen figured there were 

 six of these; their number is variable. They are the anlages of the mammary 

 glands, and are at this stage merely the local thickenings of the ectoderm or 

 epidermis. There has been a considerable growth of the dorsal region of the 

 body, and this is perhaps most clearly indicated by the position of the milk-line, 

 which is much further away from the median dorsal line than in the 10 mm. pig- 

 The limbs are both paddle-shaped, and, though still very short, have a broad 

 terminal expansion, which is the anlage of the foot. The front foot has some- 

 what the outline of a truncated pyramid, while the hind foot is more rounded. 

 In the anterior limb the differentiation into upper and lower divisions is sug- 

 gested. 



Pig Embryo of 20 mm. As compared with figure no this stage (Fig. in) 

 shows a general progress, but no such striking changes of external form as 

 distinguished the embryo of 15 mm. from that of 10 mm. Embryos of this 

 length vary considerably in their proportions, but the one figured is charac- 

 teristic of this stage. The enormous transverse diameter of the body as 

 compared with its length is very striking, and the very large size of the head 

 as compared with the body is almost equally remarkable. In the head the 

 growth of the regions of the 'fore-brain and mid-brain has continued, and the 



