244 MARY BLOUNT. 



the blastoderm at 4:25 P. M., or twenty and a half hours after 

 fertilization. Some of these cells at least have been derived by 

 division of such a marginal blastomere as shown in Fig. 9. 

 Others may have been derived from the periblast, with nuclei 

 sisters to those yet remaining in the unsegmented part. 



Fig. 1 1 is the posterior end of a longitudinal section through 

 an egg perhaps twenty-five hours after fertilization (8:50 P. M.) 

 Four nuclear nests and two single nuclei are found in the peri- 

 blast. Beyond the limits of the drawing, are four other nuclei 



Fig. 11. Posterior side of a longitudinal section of a pigeon's egg about twenty- 

 five hours after fertilization, 8:50 P. M. I. Nests of periblast nuclei. 2. Periblast 

 nucleus. 3. Syncytial mass derived from the periblast, organizing into cells which will 

 be added to the blastodisc. 4. Vacuoles. 



two of them are in line with the most extreme nucleus to the left 

 and two are a little deeper. Large masses, as shown at 3, Fig. 

 11, are organized out of the periblast and subsequently they 

 divide into smaller cells. Indentations just to the left of the 

 segmented part here suggest future cleavage which would add 

 superficial cells. (Compare Fig. 14.) This figure (Fig. 11) re- 

 sembles Harper's (3) Fig. 36 which is a section of an egg fifteen 

 hours after fertilization. Harper considers that the " free nuclei " 

 are sperm nuclei but there was a gap in his material just at the 

 period when the sperm nuclei disappear and the periblast is 

 organized. The nuclei of his Fig. 36 are doubtless periblast 

 nuclei. 



Fig. 12 shows a marginal part of a horizontal section through 

 an egg of the same age as Fig. 1 1 (twenty-five hours after fertili- 

 zation, 8:50 P. M.). Here are "free nuclei" or periblast nu- 



