222 



STUDY OF PIG EMBRYOS. 



The length of the embryo measured in a vertical line as the embryo is placed 

 in the figure is 7.5 mm., but its greatest length in any direction is 8.0 mm. 



The head is somewhat triangular in form, being broadest toward the front (the 

 left in the figure) and narrowing posteriorly to join the rest of the body. The 

 upper boundary of the head is a nearly straight line, the extent of which marks 

 approximately the territory of the hind-brain. 



Toward the left the outline forms a rounded curve which marks the territory 

 of the mid-brain, and then continues obliquely downward in a straighter course un- 

 til it curves over on to the under side where it forms three notches. The first 



notch indicates the position of the mouth, the 

 second marks the boundary between the first and 

 second branchial arches, the third the boundary 

 between the second and third arches. On the tip 

 of the head, just in front of the mouth, is a shal- 

 low depression, the anlage of the nasal pit, and 

 above is the small eye. From the eye to the mouth 

 runs a shallow furrow, the lachrymal groove. The 

 first branchial arch is called the mandibular; it is 

 broad and separated by a furrow from the second. 

 Between it and the eye lies the maxillary process. 

 The second branchial arch is termed the hyoid. 

 The third is smaller and somewhat drawn inward, 

 while the fourth and fifth have sunken so far as to 

 produce a deep pit with a triangular outline, which 

 has been named the cervical sinus. 

 The body has a long curving dorsal outline terminating in the recurved tail. 

 Near this outline thirty-seven segments show externally, because each one creates 

 a protuberance of the ectoderm. The least developed segments are in the tail. 

 From there toward the head they show a progressive advance in the stage of devel- 

 opment attained. The two limbs are rounded buds, the anterior being the larger, 

 and offer no trace of their future articulation. Between them stretches a long 

 protuberance, which is due to the Wolffian body or mesonephros, the precocious 

 development of which is characteristic of ungulates. In man at a corresponding 

 stage the mesonephros is relatively less voluminous. Immediately ventrad from the 

 fore-limb the two lobes of the liver can be discerned through the translucent body- 

 walls. Between the liver and the head is the very large heart. The division be- 

 tween its auricle above and its ventricle below can be seen clearly. The abdominal 

 region of the body is prolonged outward between the tail and the heart, and so 

 forms the commencement of the umbilical cord, the end of which is marked by 

 a thin membrane, the amnion, which has been almost completely removed. In 

 life the amnion forms a closed sac around the embryo, and is distended by the 

 amniotic fluid. From the end of the umbilical cord project remnants of the yolk- 



FIG. 165. PIG EMBRYO OF 7 .5 MM. 

 X 8 diams. 



