540 Stewart Paton 



and then on its return to the middle line is prevented, by means of 

 a DcecUe whose point is covered with blotting paper, from passing 

 outwards to the left, it will be noticed that on the next outward 

 excursion to the right the distance covered by both head and tail 

 is shorter than would be the case if the swing to the left had been 

 uuinterrupted. 



In the diagram the line Ä — B represents the median line of the 

 body, the curve C — D the outer limit of the unimpeded excursion to 

 the right, while the dotted line E — F represents the 

 ^ ^ ^ extent of movement when the passage of head to the 

 left is prevented. After careful study of these move- 

 ments the Impression is strengthened that the diffe- 

 renee between the two outward excursions. of the head 

 to the right caunot be explained simply as the result 

 of the body gaining greater impetus iu the swing when 

 the movement to the left is unimpeded, and therefore as 

 a result passing further out to the right on its return. 

 The explanation of this phenomenou is to be sought 

 for in the interference caused to the whole movement 

 by an obstriiction acting at the opposite end of the 

 body. Generally as the embryo grows the interference 

 B ^ D with the spontaneous movements of one end is associa- 

 t0d less and less with corresponding disturbances 

 in those of the other. At first the general radiation of Impulses 

 causing contractions is seriously impaired by the presence of any 

 Opposition, even if acting upon a very remote point. Graduai ly 

 the extent and strength of the movements described beeome more 

 marked, depending to some degree upon a variety of physical con- 

 ditions in addition to those associated with the growth of the em- 

 bryo. The temperature of the tluids in which the embryo is ])laced, 

 as well as the differences in chemical Constitution undoubtedly bave 

 a decided influence although unfortunately no exact data in refe- 

 rence to these points may be given. 



It is hardly necessary to mention that the tendency shown by 

 the embryo, even in its earliest stages of development to lic upon 

 its belly, as does the adult fish, is simply the result of the naturai 

 Position upon the a^^. In embryos of 13 mm. or 14 mm., as soon as 

 the attachment which holds them in position is severed, they fall at 

 once upon their side, but occasionally display a tendency to right 

 themselves when swimming. These early attemps while in motion 



J 



