Gastrulation in the Pigeon's Egg. 67 



tion suggested to me by Professor Whitman, to whom I am indebted, 

 not only for scholarly criticism, but also for his inspiring ideals 

 of research. This paper is one of the series designed by Professor 

 Whitman for the purpose of giving an account of the Natural History 

 of Pigeons.^ I also wish to express my gratitude to Professor F. R. 

 Lillie for his assistance, and to the other members of the department 

 for their interest in the work. 



II. Material and Methods. 



For the purpose of these studies the egg of the common pigeon 

 offers several advantages over that of any other bird. (1) Its small 

 size makes it especially easy to handle in preparing sections. (2) The 

 fact that this bird breeds readily in confinement renders it possible 

 to secure absolutely fresh material. (3) Undoubtedly the greatest 

 advantage, however, is that of being able to secure all the early 

 stages of development in definite sequence. This is made possible 

 by the regularity of the laying habits of the pigeon, which ordinarily 

 lays two eggs for each sitting. The first is laid late in the afternoon, 

 usually between four and six p. m., and the second between one and 

 two p. m. on the second day following. Harper ('04) has shown 

 that fertilization in the latter egg occurs shortly before it enters the 

 oviduct, about four hours after the first egg is laid, that is, at about 

 eight p. m. The second egg is, therefore, forty-one hours in passing 

 down the oviduct. Hence, by killing birds at various hours in the 

 interval between the two eggs a close series of developmental stages 

 can be secured. Such a series is indispensable to the discovery of 

 demonstrative evidence of gastrulation and to a correct interpreta- 

 tion of the attendant phenomena. 



In fixing the egg I have followed the method employed by Harper, 

 in that the whole yolk is fixed and hardened before any attempt 

 is made to cut out an oriented block of yolk containing the blasto- 

 derm. For fixing, various reagents have been employed, but the 

 picro-acetic mixtures have proved superior to all others and during 



•The series so far embraces the following: Guyer ('00), Harper ('04), 

 Blount ('07), Patterson ('07 b), Riddle ('08). 



