EARLY DEVELOPMENT OF THE PIGEON S EGG. 



243 



The transverse section represented in Fig. 9 is through about 

 the center of the blastoderm of a'n egg taken from the bird at 

 10:30 A. M., or about fourteen and a half hours after fertiliza- 

 tion. The center of the blastodisc has become three cells deep, 

 and is separated from the yolk by a sharp line bounding the 

 latter. But the marginal cells are continuous with the yolk, and 



m 



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.3* 

 i & * a. * 



k* 







*. * »• •.*. * 



Fig. 9. Transverse section through the center of the blastoderm of a pigeon's 

 eggtakenat 10:30 A. M., 14^2 hours after fertilization. 1. Marginal cell. 2. Mar- 

 ginal periblast. 3. Nuclei in the central periblast, derived from the nucleus of the 

 marginal cell. 



protrusions from the central periblast extend into the segmenta- 

 tion cavity. Nuclei are often found in these protrusions, which 

 suggest that cells are being added to the segmented part. This 

 egg is still in the cleavage stage, being twenty to twenty-two 

 hours before gastrulation [considering the time of gastrulation 

 five to seven hours before laying as determined by Mr. Patter- 

 son (7)] , and may therefore be considered not unlike the teleost 

 egg. But in following through the successive later stages, simi- 

 lar relations are found between periblast and blastodisc and there 

 is no time when they are distinct. 



Fig. 10 shows in mere outline the conditions at the margin of 



Fig. 10. Margin of a transverse section of a pigeon's egg about twenty and a 

 half hours after fertilization, 4:25 P. M. p.n, periblast nuclei. 



