THE ANATOMY OF A SIX MM. PIG EMBRYO 



91 



cociously developed in comparison with a human embryo ot the same size (Fig. 

 94). In a human embryo 7 mm. long the head is larger, the tail shorter. The 



Spinal 



Cervical segment 8 



lencephalon 



Mesencephalon 



Future milk 



Yolk sac ana 

 umbilicai cord- 



Thoracic segm 



Lumbar segment 5 



Fig. 94. — A human embryo 7 mm. long, viewed from the right side (Mall in KoUmann). X 14. 

 I, II, III, Branchial arches 1, 2, and 3; H, Ht, heart; L, liver; L', otic vesicle; R, olfactory placode; Tr, 

 semilunar ganglion of trigeminal nerve. 



cervical flexure is more marked, the olfactory pits larger and deeper. The Hver 

 is more prominent than in the 6 mm. pig, the mesonephros and segments less so. 



Dissections of the Viscera 



To understand the sectional anatomy of an embryo, a study of dissections 

 and reconstructions is essential. For methods of dissection see p. 137, Chapter 

 VI. Before stud3dng sections, the student should become as well acquainted 

 as possible with the anatomy of the embryo and compare each section with the 

 figures of reconstructions and dissections. 



Nervous System. — Fig. 95 shows the central nervous system and viscera 

 exposed on the right side of a 5.5 mm. embryo. The ventro-lateral wall of the 

 head has been left intact with the lens cavity, olfactory pit, and portions of the 



