228 STUDY OF PIG EMBRYOS. 



assume its permanent pointed form. The limbs have increased considerably in 

 length, but not yet enough to project beyond the abdomen. In both feet the 

 differentiation of five toes is clearly indicated. The milk-line, as a line, has almost 

 completely vanished, but the row of dots, the 'anlages of the mammary glands, 

 which develop along the milk-line, persists" and will undergo further development 

 in later stages. The number in the specimen figured is five. As the row of 

 anlages marks the position of the milk-line, it is readily seen that the line has 

 migrated ventralward as compared with earlier stages. Comparison of the embryos 

 of 15 and 20 mm. demonstrates that, during the period comprised between the two 

 stages, the growth of the dorsal part of the embryo- is far greater than of the 

 ventral part. Comparison of figures 167 and 168 shows at once that the area 

 occupied in both figures by the region on the ventral side of the milk-line is 

 about the same. In the pig of 20 mm. there is no indication of the segmental 

 structures recognizable in the surface modeling. 



Pig Embryo of 7.8 mm. General Anatomy. 



Anatomical Reconstructions from the Sections. Reconstructions are of the greatest 

 assistance in the study of sections, and much facilitate the identification of all the 

 parts. Students using this book should, while examining their sections, constantly 

 refer to the reconstructions. It is unnecessary to give elaborate descriptions of 

 each of them, since the explanations of the lettering of the figures will suffice for 

 the identification of all the parts shown. Certain brief explanations as to each of 

 the figures are, however, desirable. The chief value of reconstructions is to render 

 clear the topographical distribution of the organs. 



The reconstruction* presented in figure 169 shows the general topography of the 

 embryo, and illustrates chiefly the digestive, vascular, and central nervous systems. 

 The digestive tract is represented by the outline of its epithelial portion only. In 

 the actual specimen the walls are thicker, because they include the mesenchyma 

 immediately surrounding the epithelium. The mouth is an opening between the 

 head and mandible, Md, and leads directly into the pharynx, Ph. From the dorsal 

 side of the mouth springs the hypophysis, Hy, which lies close against the wall of 

 the fore-brain and is destined to form the anterior lobe of the pituitary body. 

 The pharynx, Ph, is narrow in its dorso-ventral diameter, and is represented in 

 median section. From its dorsal surface arises the small conical diverticulum, 

 S.P, situated near the hypophysis. If followed backward, the pharynx is found to 

 bend tailward and to form two median branches, the more dorsal of which is the 

 oesophagus, (E. The ventral branch is the trachea which soon bifurcates to form 

 the main bronchi, of which the right only is shown in the figure, at Bro. The 

 lateral gill-pouches are not shown in this figure. The oesophagus has lengthened, 

 and leads to the stomach, St, which has already descended and has so revolved 



* Through the kindness of Dr. Thyng, it is possible to use this illustration in advance of its publication 

 by the author, whose paper on the anatomy of the 7 .8 mm. pig is soon to appear in full. 



