EMBRYOLOGY OF CRYPTOBRANCHUS 523 



111 this stage practically all embryos are found lying on either 

 the right or the left side. To determine the relative number of 

 cases of each, one hundred and nine embryos, taken at random 

 from several different spawnings, were examined ; fifty were found 

 to lie upon the right side, fifty-nine upon the left side. These 

 numbers are approximately equal, hence the occurrence follows 

 the laws of chance. 



Rotation of the embryo now occurs in a greater proportion of 

 Qases than in the preceding stage, though of the embryos studied 

 hardly more than one in ten has been observed to rotate. It 

 is noticeable that in some cases the direction of rotation is clock- 

 wise, in other cases anti-clockwise. In eleven embryos that 

 showed rotation eight moved in an anti-clockwise direction; all 

 these were lying on the left side. Three embryos rotated in a 

 clockwise direction; of these two were lying on the right side, 

 one on the left side. Hence as a general rule the direction of 

 rotation is correlated with the position of the body. The actual 

 direction is not what one would expect if the beat of the ciha 

 were directly backward in all parts of the body; but by an inspec- 

 tion of figure 190 it will be seen that in the head region the beat 

 is ventrad, in the posterior region dorsad. The ciliary currents 

 are alike on the two sides of the body. 



Stage 20: {figs. 192 and 2/^9 to 251). This stage is reached 

 about two days later than the beginning of Stage 19. Through 

 the loss of the cervical flexure the head is now brought more nearly 

 in line with the body, but on the other hand there is an increase 

 ill the cephalic flexure. Five gill invaginations are usually visible, 

 the two posterior ones being sometimes indistinct. There are 

 distinct nasal pits. The roof of the medulla is becoming thin and 

 transparent. From twenty-five to thirty mesoblastic somites 

 are apparent in surface views. The tail undergoes a decided 

 increase in size, and is slightly flexed ventrally. The front limb 

 anlagen appear during the latter part of this stage. 



The lateral vascular bands are not yet differentiated into veins, 

 but during the latter part of this stage some small veins have 

 been observed, in living material, extending vertically from the 

 vascular bands for a short distance above them (fig. 192). No 



