CHAPTER V 



THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



A. THE ANATOMY OF A SIX MM. PIG EMBRYO 



Very young pig embryos of the primitive streak and neural fold stages have 

 been seen already (Fig. 26). In its early stages the pig embryo is flattened out 

 on the surface of the yolk sac Kke a chick embryo (Fig. 92), but as the head and 

 tail folds elongate the body becomes 

 flexed and twisted spirally, making it 

 difficult to study. In embryos 5 to 

 7 mm. long the twist of the body 

 begins to disappear and its structure 

 may be seen to better advantage. 



External Form of 6 mm. Em- 

 bryo. — ^When compared with the 

 form of the 4 mm. human embryo, 

 the marked difference in the 6 mm. 

 pig is the convex dorsal flexure which 

 brings the head and tail regions 

 close together (Fig. 93). The ceph- 

 alic flexure at the mesencephalon 

 forms an acute angle and there is a 

 marked neck or cervical fl,exure. As 

 a result, the head is somewhat tri- 

 angular in form. The body is bent 

 dorsad in an even convex curve 

 and the tail is flexed sharply dorsad. 

 Lateral to the dorsal line may be seen the segments, which become larger and more 

 differentiated from tail to head. At the tip of the head a shallow depression marks 

 the anlage of the olfactory pit. The lens vesicle of the eye is open to the. exterior. 

 Caudal to the eyes at the sides of the head are four branchial arches separated by 

 three grooves, the branchial clefts. The fourth arch is partly concealed in a tri- 



x^ 



Fig. 92. — Pig embryos {A) of seven and (B) of 

 eleven primitive segments, in dorsal view with amnion 

 cut away (Keibel, Normentaf el) . X 20. 



