1 62 STUD Y OF PIG EMBR 1 'OS. 



presenting at this stage a triangular outline; within the sinus are found the 

 external or ectodermal terminations of the second and third gill clefts. The ter- 

 ritory of the mandibular process and cervical sinus corresponds to the pharyngeal 

 region. It is the site of some of the most important, interesting, and compli- 

 cated developments by which the embryonic is changed into the adult anatomy. 

 The body of the embryo shows the position and number of the segments, 

 Seg, by the external modeling. Both limbs are well advanced, the anterior, 

 A.L, more so than the posterior. From the base of the front to the base 

 of the hind limb extends the milk-line, M.L, curving so as to be nearly parallel to 

 the dorsal outline of the body. Along this line the mammary glands are ulti- 

 mately developed. Extending across the body are several shadowy lines, shim- 

 mering through the translucent body-walls. One marks the position of the 

 embryonic diaphragm ; it extends from the upper edge of the anterior limb ob- 

 liquely downward toward the edge of the umbilical cord. Another, which 

 extends in a nearly straight line from limb to limb, marks the ventral edge of the 

 large Wolffian body or mesonephros, the dorsal limit of which is approximately 

 indicated by the milk-line, M.L. The outlines of the smaller left dorsal lobe of 

 the liver are distinct, and mark out a pointed area immediately below the fore- 

 limb, A.L. 



Pig Embryo of 12 mm. General Anatomy. 



Anatomical Reconstruction from the Sections* — The four figures herewith 

 presented are based upon the same series of transverse sections (No. 5) from 

 which figures 113 to 121 were drawn. The reconstructions have been made by 

 the method described in Chapter VIII. The actual drawings, especially as re- 

 gards their shading to indicate the modeling of the surfaces, have been partly 

 made from a wax model of the brain and a wax model of the cavity of the 

 pharynx of the same embryo. The umbilical cord of the embryo used having 

 been damaged, the loop of the intestine in the umbilical cord has been added by 

 a reconstruction from another scries (No. 518) of an embryo of the same size. 



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 expla- 

 nations 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. 



*The four reconstructions were made by Dr. F. T. Lewis, to whom the whole credit for them belongs. 

 The two wax models referred to were made by Dr. John I.. Iiremer. The author hopes that both of these investi- 

 gators will publish their results in full elsewhere. 



