2l8 W. M. SMALLWOOD. 



are visible only with the oil immersion lens. This means that 

 but few eggs will be cut in just the right plane to enable one to 

 make out the correct relations ; and it also means that important 

 conditions will escape detection. 



The first division of the egg into the two-cell stage is preceded 

 by the formation of a definite mitotic figure after the male and 

 female pro-nuclei have come together. The chromatin becomes 

 more responsive to stain and gradually condenses into a definite 

 number of very minute chromosomes. These chromosomes in 

 size and staining reaction are so similar to some of the granules 

 in the cytoplasm, Fig. 8, PI. II., that it is impossible to be certain 

 that they are chromosomes unless the spindle fibers are present. 

 This makes the determination of the number of chromosomes 

 difficult because when one has a cross-section of the metaphase 

 through the equatorial region, one cannot determine the relation 

 of the spindle fibers to the chromosomes and so cannot be certain 

 of their number. The task was a little easier in Pcnnaria, where 

 fourteen were counted in the anaphase, although here I am not 

 certain that this is the correct number. I think that there are 

 from twelve to sixteen chromosomes present in these hydroids, 

 the exact number remains to be determined. 



The chromosomes form in the typical metaphase condition, 

 split and move toward each pole of the spindle. In the anaphase 

 distinct interzonal fibers are present. During the telophase the 

 chromosomes are transformed into a nucleus. This nucleus does 

 not necessarily assume the rounded outline but is often elon- 

 gated and even irregular in shape. The prophase of the next 

 cleavage frequently begins while the nucleus is in this condition, 

 Fig. 7, PI. I. In Fig. 7 a typical prophase of mitosis in cleavage 

 is shown. The faint asters are on opposite sides of the elongate 

 nucleus, and a few spindle fibers are forming preparatory to the 

 metaphase and the dissolution of the nucleus. The centrosphere 

 as shown in Figs. 7 and 8 will be discussed in a separate section. 



A definite mitotic figure has been made out in all of the early 

 stages of segmentation and followed up to the planula stage. A 

 typical condition of the early embryo is shown in outline in Fig. 

 9, PI. II. The cells surround a cavity which is first seen in the 

 two-cell stage and is due to the separation of these first two cells. 



