226 STUDY OF PIG EMBRYOS. 



has, moreover, risen so that the neck-bend is diminished. The limbs are beginning 

 to show the differentiation 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 relatively larger 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 ap- 

 pears 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 primitive 

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

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

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

 do not extend so far on the dorsal side as in the earlier embryos, but are restricted 

 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 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 farther away from the median dorsal line than in the 10 mm. pig. Both 

 limbs are paddle-shaped, and, though still very short, have a broad terminal ex- 

 pansion, which is the anlage of the foot. The front foot has somewhat 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 suggested. 



Pig Embryo of 20 mm. (Fig. 168). Comparison of this stage with figure 167 

 reveals a general progress, but no such striking changes of external form as distin- 

 guished the embryo of 15 mm. from that of 10 mm. Embryos of this length vary 

 considerably in their proportions, but the one figured is characteristic of the stage. 

 The enormous transverse diameter of the body as compared with its length is very 

 striking, and the very large size of the head in proportion to 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 divisions between the mid-brain and hind-brain 

 are marked by concavities in the outline of the head. The eye is both absolutely 

 and relatively larger. Above it can be distinguished readily the anlages of the great 

 bristles which develop over the eye, corresponding to the human eyebrow. These 

 anlages appear as whitish spots, for they are thickenings of the ectoderm. The 

 snout has increased in length; the external ear has grown longer and has begun to 



