OF THE TADPOLE. 115 



changes whereby the undifferentiated yelk-mass is converted into 

 a spiral intestine. It must be remembered that this yelk-mass, 

 after the formation of the viscera (already mentioned in the 

 previous articles), consists of an egg-shaped mass, the larger end 

 being towards the front and the smaller somewhat elongated end 

 behind. Through this smaller end the anal portion of the 

 canal extends, well marked by dark cells arranged close together 

 in two parallel rows, leaving a space between them. The first 

 alteration appears to take place in the upper portion of the broad 

 anterior end, which becomes indented and gradually turned 

 round, so as to form a kind of crook, whose cells arrange them- 

 selves in parallel rows, leaving a mass of less-altered cell-tissue 

 between them, thus producing a resemblance to a closed tube. 

 Next, or almost simultaneously, the yelk-rnass breaks up into two 

 l)ortions, each surrounded by its own delicate membrane. From 

 the upper mass, the greater portion of the coiled intestine is 

 formed, whilst the lower is converted into the bladder. This 

 conversion of a mass of cells into a tube takes place by certain 

 cells parting slightly from each other, and their inner edges 

 becoming thicker, blacker, and more dense, thus forming the 

 rudiments of a tube. It is at the front portion of the yelk that 

 the tubular structure is first seen, and as these portions become 

 separated from each other they are enveloped with a very delicate 

 skin, which ultimately forms the mesentery, and holds the whole 

 of the spiral canal in place. The bladder, at one period of its 

 history, appears as a hollow mass of cells bounded by conical 

 masses, with their apices pointing inwards. 



After the indentation and marking-off of the anterior portion 

 of the yelk-mass, the lower part also becomes tubular, so as to 

 attain a form somewhat like that shown in Plate XL, Figs. \a, id, 

 ic. From this point, it is almost impossible to follow the exact 

 stages whereby this mass becomes formed into a spiral. The 

 proctodeum, however, is soon connected with the inner coil, and 

 it only appears to be towards the last stage of the process that 

 the mouth becomes attached and a perfect tube is thus formed. 

 Some of the stages are shown in Plate XL, Figs. 2, 3, and 4, and 

 the perfectly coiled tube in Plate XII., Fig. 5, where the coil 

 apparently takes up the whole of the lower portion o( the abdo- 



