FOURTH DAY EMBRYO. 451 



2. The second and third visceral arches and clefts. 



3. The nasal pits. 



F. Sections. Manipulation as in I. B. 3. 

 The most important sections are : 



1. Through the eyes in the three planes, vide Fig. 50, 

 A. B. C. 



2. Through the auditory sac. 



3. Through the dorsal region, shewing the general 

 changes which have taken place. 



Amongst these, notice 



a. The changes of the mesoblastic somites: the com- 

 mencing formation of the muscle -plates. 



b. The position of the Wolffian duct and the forma- 

 tion of the germinal epithelium. 



c. The aortce and the cardinal veins. 



d. The great increase in depth and relative diminu- 

 tion in breadth of the section. 



V. Examination of an Embryo of the Fourth Day. 



A. Opening the egg, as in II. A. 



Great care will be required not to injure the 

 embryo, which now lies close to the shell-membrane. 



B. Examination in situ. Observe: 



1. The now conspicuous amnion. 



2. The allantois, a small, and as yet hardly vascular 

 vesicle, beginning to project from the embryo into 

 the space between the true and the false anmion. 



3. The rapidly narrowing somatic stalk. 



292 



