The Early Development of the Pigeon's Egg. 17 



from that which exists in earlier stages, in which the supernumerary 

 sperm nuclei are the cause of the accessory cleavage. This is a 

 stage after the disappearance of the sperm nuclei. The marginal cells 

 are continuous with the periblast; the nuclei in the periblast are 

 sisters to the nuclei of the marginal cells, i. e., they are derived from 

 the cleavage nucleus ; and the small cells resembling accessory cleav- 

 age are mere bud-like and evanescent projections from the periblast. 

 In no section of this egg are there found the diagonal, submarginal 

 cleavage planes like those which separate the primary area from 

 the region of sperm nuclei as shown in Chart IV. 



Fig. 11. — Transverse section through the center of the blastoderm of a 

 pigeon's egg taken at 10.30 a. m., 14i^ hours after fertilization. 1. Marginal 

 cell. 2. Marginal periblast. 3. Nuclei in the central periblast, derived from 

 the nucleus of the marginal cell. 



Chart VII. 



An egg taken from the bird at 10.30 a.m., fourteen and one-half 

 hours from the estimated time of fertilization, furnishes the data 

 for Chart VII. Although obtained at the same hour as the egg for 

 Chart VI, it was probably fertilized earlier ; for it shows a later stage 

 of development. Chart VII A is a free hand drawing, but a photo- 

 graph of another egg of about the same stage is shown in Fig. 54. 

 At this stage the peripheral limit of the periblast does not show in 

 surface view. In sections of the egg, however, nuclei are found in 

 marginal and central periblast — see Chart VII B and C, and also 

 Fig. 11, which represents a central transverse section of the egg 

 of this chart. From this figure it is plain that the marginal cells 

 constitute a "zone of junction" between the blastodisc and the peri- 

 blast Agassiz and Whitman, '84). 



