312 Mr MacBride, The Relationship of [May 24, 



ventral part does not become confluent with the ventral part of 

 the trunk coelom. The ventral part of the collar coelom grows 

 back externally to the trunk coelom, forming two ventral lateral 

 ridges which reach some little distance behind the last gill-slit 

 which is formed. These ridges, which are traceable as soon as 

 the mouth and first gill-slit have been formed, are really the 

 first rudiments of the atrial folds, which later hang down over 

 and cover the gill-slits. The ccelom contained in these ridges 

 gives rise to the ventral muscle of the atrial cavity. Now in 

 Balanoglossus the hinder end of the collar extends over and covers 

 the first gill-slit, and the suggestion of Bateson that this re- 

 presents the first beginning of the formation of an atrium like 

 that of Amphioxus is borne out by the facts I have just narrated. 



I have said above that it was difficult to suppose that active 

 Vertebrates were descended from a sluggish worm like Balano- 

 glossus. It is however unnecessary to make such a supposition: 

 when zoologists point to one existing animal as "representing" 

 the ancestor of another, they often use loose and inaccurate 

 language. 



The evidence that Lwoff brings forward in support of his 

 position is exceedingly feeble. The first shape which the de- 

 veloping egg of Amphioxus assumes is that of a hollow sphere — 

 the so-called blastula: this then becomes converted into the 

 well-known gastrula by the process of invagination, one half being 

 pushed inside the other. Now Lwoff maintains that in the 

 blastula certain cells are distinguished from the rest by their 

 size, and that these alone represent the endoderm of other 

 animals, they are first invaginated, and then the invagination of 

 cells which are really ectoderm succeeds to this. Considering 

 that the invagination is a continuous process which proceeds 

 regularly to its completion, and that when it is completed no 

 difference in the character of the invaginated cells can be detected, 

 it will be seen that this is a most violent and arbitrary interpre- 

 tation of the facts, which could only be entertained if it were 

 shown that in the blastula the endoderm was clearly and sharply 

 defined. I have cut about a hundred of invaginating blastulse to 

 examine this point and can meet it with a direct denial. 



It is true there are certain cells of the blastula distinguished 

 by their larger size from the rest, but these merge gradually into 

 the rest at the sides, and where the process of invagination just 

 shows itself the smallest cells are found. This shows that the 

 size of a cell depends on the rapidity with which division is going 

 on, and as the process of invagination is connected and in fact 

 caused by division and multiplication, it is natural that smaller 

 cells should be found at the edges of the area which is about to be 

 invaginated. 



