CHAPTER V 

 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



A. THE ANATOMY OF A 6 MM. PIG EMBRYO 

 In its early stages the pig embryo is flattened out on the surface of the yolk- 

 sac like a chick embryo (Fig. 87), but as the head- and tail-folds elongate the body 

 becomes flexed and twisted spirally, making it difficult to stud)-. In embryos 

 5 to 7 mm. long the twist of the 



body begins to disappear and its B 



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, 

 its marked difference is the convex 

 dorsal flexure which brings the 

 head and tail regions close together 

 (Fig. 88) . The flexure at the mesen- 

 cephalon forms an acute angle and 

 there is a marked neck or cervical 

 flexure. As a result, the head is 

 somewhat triangular in form. The 

 body is bent dorsad in an even 

 convex curve and the tail is flexed 

 sharply dorsad and to the right 

 side. Lateral to the dorsal line 

 may be seen the segments, which 

 become larger and more differen- 

 tiated as we go 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 con- 

 cealed in a triangular depression, the cervical sinus (see Fig. 92). The first, or 



7 97 



Nbg 



Fig. 87. — Pig embryos (.4) of seven and (B) of 

 eleven primitive segments, dorsal view, with amnion 

 cut away (Keibel, Xormentafel). X 20. 



