30 



COMPARATIVE ANATOMY 



CHAP. 



1. An outer yolk membrane, secreted by the yolk itself. 



2. The formative yolk, a small whitish mass in which lies the germinal vesicle 

 at one pole of the egg, viz. the animal pole. 



3. The yellow nutritive yolk, which represents the principal mass of the egg, 

 and wluch appears in concentric layers. 



4. The white nutritive yolk, which forms a thin outer layer round the yellow 

 yolk beneath the yolk integument and the formative yolk, and which also sinks into 

 the middle of the yellow yolk from the animal pole in the form of a thick strand with 

 a swollen rounded end. 



"When the egg thus formed passes out of the ovary into the oviduct, the walls of 

 the latter secrete around it the last envelopes, which are : 



1. The albumen with the chalaza, i.e. somewhat denser spirally twisted 



Pig. 26.— Diagrammatio longitudinal section of anewly-laid. Hen's egg, after Allen Thomson 

 and O. Hertwlg. b.l. Formative yolk; w.y. white yollt ; y.y. yellow yollc ; w. albumen; ch.L 

 chalaza ; a.ch. air-chamber ; i.s.m, inner ; s.m. outer layer of the shell membrane ; s. shell, v.t. yolk 

 membrane ; x. a somewhat fluid layer of albumen surrounding the yolk. 



strands of albumen, which go from the yolk membrane out towards the two ends of 

 the egg. 



2. The double shell integument, which surrounds the albumen on all sides. 

 The two membranes which form this integument separate, and between them a cavity 

 filled with air arises at the blunt end of the egg — the air chamber. 



3. The porous calcareous shell. These three parts, therefore, represent the 

 accessory integuments of the egg. 



Male Reproductive Cells, op Spermatozoa (Fig. 27). 



These belong to the smallest cells formed in the animal body. Each 

 spermatozoon is a simple cell, as is shown by its development which is 

 generally complicated, and is very difficult to observe. A very common 

 form of the spermatozoon is the so-called pin-shaped. Such a sperma- 

 tozoon consists of a small knot, the head, representing the remains of a 



