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 like 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. The an- 

 atomy of a 7.8 mm. pig embryo 

 has been studied by Thyng 

 (Anal. Record, vol. 5, 1911). 



External Form of 6 mm. 

 Embryo. When compared with 

 the form of the 4 mm. human 

 embryo, the marked difference 

 in a 6 mm. pig is the convex 

 dorsal flexure which brings the 

 head and tail regions close to- 

 gether (Fig. 93). The cephalic 

 flexure at the mesencephalon 

 forms an acute angle and there 

 is a marked neck, or cervical 



FIG. 92. Pig embryos, (A) of seven and (B) 

 of eleven primitive segments, in dorsal view with 

 amnion cut away (Keibel, Normentafel). X 20. 



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. 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 branchial grooves, The fourth 



arch is partly concealed in a triangular depression, the cervical sinus. 



