CYTOLOGY OF HYDRACTINIA AND PENNARIA. 



21 I 



ing rapidly in size. In the entoderm cells it is so very small that 

 it is difficult to be sure of its presence in some cells, and the same 

 is true in some of the young ova, but in most instances it is a 

 clearly defined and easily distinguished body. 



The most interesting change is found in the chromatin network. 

 In the young ova it is now in its most conspicuous state. From 



FIG. I. Outline drawing of base of polyp to show the 

 position of the ova. ec, ectoderm ; en, entoderm; ov, 

 ova. Hydractinia. 



pnorO O 



FIG. 3. Camera lucida drawing of the egg nucleus, 

 the larger circle, and the female pronucleus, the smaller 

 circle. TV/, nucleus ; pronl. 9 > female pronucleus. 



FIG. 2. Outline drawing to show that sometimes the Hydractinian polyp branches. 

 B, base of polyp. Reconstructed from several sections. 



this time on until new ova arise in a new polyp the chromatin 

 does not possess such distinctness. A loose network is distributed 

 through the nucleus with conspicuous masses where the threads 

 cross. The achromatic substance (residual substance of Lillie, 

 '06) does not stain in the acid or basic stains. The nuclear 

 membrane is very distinct, chiefly due to the fact that a con- 

 siderable amount of chromatin material seems to be directly in 

 contact with it. The cytoplasm is as yet free from the micro- 

 somes --minute granules which characterize the slightly older 

 ova. 



The next step in this gradual transformation is the rapid in- 

 crease in the cytoplasm, accompanied by a growth in the nucleus. 

 The cytoplasm is now loosely sprinkled with microsomes, in 



