BONES, JOINTS, CONNECTIVE TISSUES 33 



the length of the oviduct, there occurs only three successive mitotic 

 divisions, including the first segmentation division, namely mitoses re- 

 sulting in 2-cell, 4-cell, and 8-cell stages while the ova are in transit in 

 the oviduct. In making this statement it is assumed that in the suc- 

 cessive segmentations, the several cells divide synchronously, which is 

 not in conformity with the fact. These three mitotic divisions are 

 spaced at intervals of about 18 hours. 



In the next following division, the fourth, the ovum passes from the 

 oviduct to the uterine horn. Since the normal gestation period of the 

 non-lactating albino rat is only 21 to 23 days, this slow rate of increase 

 in volume and multiplication of cells during the first four days of de- 

 velopment is of especial interest and is very probably to be accounted 

 for by the inadequacy of the food supply of the ovum during its transit 

 through the oviduct. 



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

 burg (MS '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. 64 and 65. 



3. Bones, joints and connective tissues. On the following page 

 is an enumeration of the bones forming the skeleton of the rat. 



For data on the growth of the entire skeleton see Chapter 6. 



Skull measurements have been made by Hatai ('07 c) . The 

 following description is extracted from his paper. 



For this study 53 male and 51 female skulls of mature Albinos (rats 

 more than 150 days old) were measured. These skulls had been care- 

 fully cleaned and dried at room temperature. The following measure- 

 ments were made with vernier calipers: 1) the length of the entire 

 skull; 2) the fronto-occipital length; 3) the zygomatic width; 4) the 

 length of the nasal bone; 5) the height of the skull; 6) the width of 

 the cranium or the squamosal distance. In every case the maximum 

 length alone was recorded in millimeters. 



The horizontal straight line joining the tip of the nasal bone to the 

 end of the occipital bone is called the length of the entire skull. This 

 however is not exactly equal to tjie sum of the length of the nasal bone 

 and that of the fronto-occipital. 



The fronto-occipital length was determined in the following way: 

 Since the length measured with the calipers from the tip of the nasal 

 bone to the posterior end of the inter-parietal bone is usually less than 

 the length measured from the same point to the end of the occipital 

 bone, both measurements were taken (see fig. 1). The difference be- 

 tween these two measurements was added to the length from the tip 

 of the frontal bone to the end of the inter-parietal bone, and the sum 

 was called the fronto-occipital length. 



The width of the cranium (squamosal distance) was determined by 



