178 



THE OOLOGIST. 



the erythrismal phase, which is the 

 commonest there, although "mottled 

 owls" are frequent. I was not fortu- 

 nate enough to meet with the Florida 

 Burrowing Owl. 



The resident Meadow Larks and Red- 

 winged Blackbirds were particularly 

 small on the average, although very 

 large specimens of the former were 

 sometimes obtained. The smaller ones 

 averaged about 8.55 x 13.70 x 4 00. 

 Northern larks vary much, however in 

 siz i and brightness of the yellow 

 breast. 



N. HOLLISTER. 



Delavan, Wis. 



The Embryology of a Bird. 



It is evident at once that a subject 

 upon which alone whole volumes have 

 been written, can receive but the 

 merest outline in a sketch like the pres- 

 ent, and I can only hope that a few 

 brief facts, stated in the simplest lan- 

 guage may be of interest to those, who, 

 while not caring to enter deeply into 

 the intricate details, would still wish to 

 know the principle step in that mys- 

 terious process of development, which 

 transfoi ms the inanimate egg into the 

 living, breathing chick. 



And first a few words upon the an- 

 atomy of the unincubated egg, for its 

 two divisions of White and Yolk are 

 not quite so structureless as they at 

 first appear. The White may be easily 

 disposed of since it serves merely as a 

 store of nutriment for the growing 

 chick without taking any active share 

 in its formation. Confining our atten- 

 tion then to the Yolk we observe, first 

 that it is surrounded by a very thin, 

 structureless, but important sheath, 

 called the Vitelline Membrane and 

 from each end of this membrane ex- 

 tending through the white to the ends 

 of the egg. passes a delicate cord, which 

 while aiding to hold the yolk in place 

 allows it to rotate. The object of this 



cai'eful arrangement is saen directly 

 when we lind that upon the yolk, just 

 beneath the Vitelline Membrane lies a. 

 small mass of alluminous substance 

 called the Cicatricula which is alone th& 

 vital and essential portion of the whole 

 egg, and it is for the purpose of keep- 

 ing this important structure always 

 highest, where it niaj receive the great- 

 est amount of heat, that rotation of the 

 yolk is permitted. 



What now is the first change ccurr- 

 ing as incubation begins? The cicatri- 

 cula divides into two parts, each of 

 these immediately into two more and 

 this continues to be repeated until 

 there remaius but a collection of globu- 

 lar, microscopic bodies, all exactly 

 similar, known as cells. This is called 

 Segmentation, a process which, gener- 

 ally speaking, occurs in the ovum (or 

 "egg") of every animal from that of a 

 sponge to those of mammals them- 

 selves. Segmentation being complete, 

 fluid collects between the cells, which 

 at the same time spread themselves 

 out, disc like beneath the Vitelline 

 Membrane. This disc is the Blastoderm 

 and is the first rudiment of the embryo 

 although composed of cells which, as 

 we have said, are precisely alike, they 

 will finally change, each to its special 

 nature. This process is known as ■ 

 Differentiation, one of the f undimental 

 facts in life which science is never like- 

 ly to explain. The Blastoderm now 

 begins to curl downwards and inwards 

 at sides and ends until they meet when 

 they unite and form a sac entirely 

 closed except a communication below 

 with the remaining larger portion of 

 the yolk, through which the latter can 

 be absorbed as needed for growth. 

 Meanwhile the cells on the upper sur- 

 face of the Blastoderm on what is to be 

 the back of the embyro, are very active. 

 A groove forms in its long axis, the cells 

 on either side heap up, arch over the 

 groove, unite and form a hollow tube, 

 slightly lai'ger at one end, This is the 



