THE AXIS OF THE EMBRYO. 143 



This is not saying that the egg cannot be forced to change its 

 position within the membrane. The eggs that fail to become 

 attached to the bottom of the dish are liable to be rolled about, 

 and the movement may be sufficient to cause some shifting in 

 position. The important fact here is, that, whatever be the 

 place of attachment, whether above, below, or on the side, the 

 axis of the ^^g always maintains the constant relation of being 

 perpendicular to that point, and there is no inherent tendency 

 to assume a vertical position, as is the case in most pelagic fish 

 eggs. 



There is, then, no cause for rotation within the membrane 

 unless it be supplied artificially from without. The ^gg remains 

 in the position in which it becomes fixed, neither rotating nor 

 becoming displaced except as the result of rough treatment. 



In the experiment of '98 great care was taken to guard 

 against any possible disturbance of the eggs. The pieces of 

 glass with the adherent eggs were arranged conveniently for 

 observation before they were fertilized, and the water changed 

 from time to time by means of a siphon, so that no disturbance 

 seemed possible. 



If the Q,gg do€S not rotate during the early stages of develop- 

 ment, it may be asked, What is the nature of the adhesion that 

 prevents rotation } 



In regard to this matter Professor Whitman assures me that 

 adhesion of the yolk to the membrane is a very general phe- 

 nomenon which he has noticed in the ^^^ of Nectitrus, Am- 

 blystoma, frog, newt, pelagic fish eggs, and even in the small 

 eggs of many annelids. Speaking of pelagic fish eggs, he 

 says : " It is only necessary for an ^gg to be left at rest for a 

 few minutes in order to see how readily it adheres to its mem- 

 brane. I have often taken advantage of this adhesion to roll 

 over and hold them some moments by the aid of needles, with 

 the lower pole uppermost. Others, I am sure, have done the 

 same thing while studying the cleavage." He also adds that 

 in the case of the toadfish "the strength of the adhesion is 

 probably due to the relatively large weight of the ^g^ and the 

 large surface of contact." The perivitelline space in this q^^ 

 is so slight as to be difficult of recognition, and the extent of 



