42 ANATOMY 



(d) Later stages. Observations have been made by Stotsen- 

 burg ('15) on the daily increase in the weight of the fetus from 

 the 13th to the 22nd day after insemination. The data and 

 graph are given in chapter 5, pp. 173-174. 



W. Pyle '19 (MS.) reports that the giant cells — polykaryo- 

 cytes — appear in the liver on the 17th day of fetal life — but by 

 seven days after birth they have completely disappeared. In 

 the spleen they appear at the same time as in the liver. They 

 are abundant in the spleen during the first month after birth, 

 diminish gradually up to 100 days, but even at later ages an 

 occasional cell may be found. 



(e) Monsters. Tailless rats. Occasionally tailless Albinos are 

 born. The deficiency may be complete or partial. The In- 

 stitute records show about one in seven thousand. Conrow 

 ('15) has examined several of the complete cases and finds the 

 defect in the vertebrae usually to involve the sacral vertebrae 

 and in some cases to extend cephalad as far as the sixth lumbar. 

 In rats surgically deprived of the tail at birth (Conrow, '17) 

 the defect does not extend above the point of operation. At- 

 tempts to establish a strain of tailless rats have thus far failed. 

 The congenitally tailless rats have notably heavy hypophyses. 



Small-eyed rats. These appear now and then. This defect 

 occurs in all grades and may be bilateral or unilateral. King 

 (MS.) has obtained some evidence for the inheritance of this 

 defect. The anatomical condition points to the early forma- 

 tion of the eye, followed by involutionary changes. 



Mammary glands. There are normally six pairs, but defi- 

 ciencies sometimes occur, and Stotsenburg (MS.) has noted 

 cases where only six of the twelve nipples were present and has 

 been able to carry a deficiency through four generations. The 

 defect is in the system rather than in a particular pair or group 

 of glands, and in Stotsenburg's series is most commonly repre- 

 sented by absence of the upper pectorals. 



A six-legged rat has been described by Conrow ('17 a). 



