292 NEW YORK STATE MUSEUM 



The podocyst is now an object of great beauty, to whicli no 

 drawing can do justice. It undergoes a rhythmic movement of 

 dilatation and contraction, sometimes being so expanded as to 

 include the rest of the embryo, then contracting to less than one 

 half that size. The ectodermal sac also undergoes a similar con- 

 traction and expansion, sometimes in harmony with, and again 

 alternating with that of the podocyst. 



The whole embryo, from the stage represented by figure 11, has 

 a rotatory motion. 



In the stage represented by figure 19, I have first observed the 

 '' primitive kidneys." They are situated on each side of the en- 

 doderm, and consist of a iseries of curved elongate cells, within 

 which concretions are developed, and terminate in a duct (pi. 28, 

 fig. 4). 



The cells proper are apparently angular or of different shapes 

 (pi. 28, fig. 1, 2); but this is due to mutual pressure, as when the 

 sac is ruptured the cells fiow out perfectly spherical^ as shown in 

 figure 3. 



As development proceeds a movement of the cells takes place 

 from the ectodermal sac into the constantly enlarging body (pi. 26_, 

 fig. 1). The alimentary canal accompanies the cells; the anus 

 alone remains in its original position. 



In the stage represented by figure 5, plate 26, the ectoderm so 

 closely bounds the endoderm as not to be apparent. The ectoder- 

 mal cells have been largely absorbed into the body, or, rather, 

 changed into body tissue, and the podocyst is very much reduced 

 in size. The tentacles are assuming their mature form, and the 

 mouth and lips are very distinct. The intestine is nearly com- 

 plete and the larger endodermal cells have, in great part^, been 

 converted into the substance of the digestive gland. The respira- 

 tory orifice, the mantle and shell are well formed. 



Heretofore the embryo has been represented with the ventral 

 part uppermost, but in figure 2, plate 27, it is represented, in a 

 reverse position, as in adult condition. 



In this stage the endodermal cell mass has been almost entirely 

 changed into the alimentary canal and its appendages, the podo- 



