77 



process is on such a large scale that it can be easily followed with the 

 naked eye, such eggs presenting a striking contrast with the normal 

 forms , when the blastoderm is formed nearly simultaneously over 

 the whole surface of the egg. As the lips of the »blastopore« contract, 

 (I use the term to suggest what seems to me to be the same thing in 

 vertebrates, but without for a moment assuming that it is in any way 

 associated with a »gastrula«) they often turn inward and unite a little 

 below the surface, thus pinching off a goodly portion of the yolk from 

 the rest of the egg. In hardened eggs this isolated mass of yolk, if not 

 already completely free from the rest of the egg, may be loosened with 

 needles and removed. It leaves a great hemispherical depression on 

 the under side of the egg and lined with a layer of blastoderm cells 

 continous with those on the upper surface. 



Before the blastopore closes the exposed yolk may be coarsely seg- 

 mented as in the normal egg and the same is true of the isolated yolk- 

 plug. In most cases the lips of the blastopore contract slowly and 

 finally close at the surface without cutting off any yolk. 



The »blastopore« is often so irregular in outline that the gradually 

 closing lips may nip off some of these irregularities , thus leaving one 

 large blastopore with one or two smaller ones near its periphery. 



I have had thousands of artificially fixed eggs under observation 

 and have never seen one with a blastopore on its upper surface. On 

 the other hand, a very large percentage of the eggs that developed at 

 all, certainly something over fifty per cent., show traces of this remar- 

 kable blastopore on the undo rside. On stripping offa layer of eggs and 

 reversing them one can easily see how, as the blastopore narrows, that 

 it coincides more and more accurately with the small scar of attach- 

 ment on the chorion. There seems to be no doubt that the blastopore 

 occurring as it does at the point where the egg is fixed, is caused either 

 by constant pressure of the overlying mass of yolk, or by the lack of 

 aeration. The latter supposition does not seem very probable, Avhen 

 we consider how little the eggs of Limulus are influenced by the con- 

 dition of the water. On the other hand I do not recall having seen 

 these »blastopores« on eggs that had been raised under normal condi- 

 tions. This is somewhat surprising, for, if it is the constant pressure 

 of the overlying yolk alone which prevents the formation of the blasto- 

 derm on the under surface, we ought to find these blastopores in eggs 

 deposited naturally in the sand, for there is no reason to suppose that 

 they shift their position so as to equalize the pressure on all sides. 



While gravity thus seems to determine the position of the »bla- 

 stopore« it has no influence whatever on the position of the embryo 

 or on its planes of symmetry in reference to the blastopore. In eggs 



