for more than half the length of the bud, but I have one speci- 

 men, one just about to be detached from its parent, in which at 

 least one of the muscles may be followed for the whole length 

 of the bud from its distal to its proximal end. There is also 

 a difference between the two poles of the larva to be noticed 

 in the entoderm.. Throughout, it is a columnar epithelium, 

 but at the proximal end it is rather thin and gradually becomes 

 much t-hicker at the equator. In this proximal half the cells 

 are somewhat crowded and contain coarse granules. From the 

 equator to the distal pole the cells gradually become broader 

 and more clear, and the most distal cells are large and hyaline 



III. Formation of the Mouth . 



The first change noticeable in the swimming larva is the 

 formation of the mouth, and this does not occur until two or 

 three days after the larva has been set free. At the time of 

 its first appearance, the moutli is a minute opening in the pos- 

 terior end of the larva. Figs. 3a and b. ^UTiether this 

 point is identical with the distal, or with the proximal end 

 of the bud, it is difficult to say. The evidence points in 

 both directions. 



That this point was the distal apex of the bud seems pro- 

 bable, when we observe, in the first place, that in the comple- 



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