The Early Development of the Pigeon's Egg. 



51 



vollzieht sie sich bei denen der Knochenfische, deren Blastomeren 

 friihzeitig von korperlichen Dotterbestandtheilen frei erscheinen." 

 (His, '00, p. 187.) 



Three diagrams (Fig. 43) represent median vertical sections of 

 the egg of amphioxus, the frog, and the pigeon. In amphioxus, the 

 cells at the vegetative pole are but slightly larger than those at the 

 animal pole, and the segmentation cavity is nearly central. In the 

 amphibia, the segmentation cavity is nearer the animal pole, and 

 the vegetative cells are large, but are separated from each other. 



a 



c p 



Fig. 44. — Diagrams of transverse sections of tlie eggs of (a) ctenolabrns, 

 and (b) the pigeon. A is copied from Agassiz and Wliitmau ('84) Fig. 2. I 

 tiave inserted tlie dotted lines to indicate the region from which cells are added 

 from the periblast to the blastodisc. mc, marginal cell ; cc, central cell ; mp, 

 marginal periblast; cp, central periblast; sc, segmentation cavity. 



In the bird, the segmentation cavity is much further removed from 

 the vegetative pole, and in relation to the size of the egg it is too 

 small for comparison with the homologous cavity in the other eggs. 

 The cells of the vegetative pole are not separated from each other, 

 but they are the syncytial periblast. The cytoplasmic portion of 

 the cells is confined to the animal pole, while their yolk contents 

 occupy the entire remaining part of the egg. In Ctenolabrns cells 

 are added to the central area from the marginal cells until the end 

 of the period of cleavage (Agassiz and Whitman, '84, Fig. 5). The 



