VERTEBRATE DEVELOPMENT 233 



The portion of the Hen's egg corresponding to an entire Frog's 

 egg is the so-called yolk. In the early stages of development in the 

 ovary, the egg cells are typical in size and shape. During the 

 developmental stages a considerable amount of food material is 

 assimilated by each of these cells which are to develop into mature 

 eggs, so that they rapidly increase in size. There is a certain 

 rhythm in egg formation. The Hen usually lays an egg a day for 

 some two or three weeks, and then will stop for a period during 

 which the clutch of eggs can be incubated. This may then be 

 followed by another period of egg laying. (W. f. 169.) 



The large amount of food material present in the egg crowds 

 the protoplasm to the surface of one pole, where it can be seen in a 

 mature egg as a tiny, white, circular area, the blastoderm. The 

 remainder of the cell outside of this small blastoderm consists 

 chiefly of inert food material. This condition is an exaggeration of 

 that found in the Frog's egg, where the greater proportion of the 

 protoplasm is at the animal pole and the greater proportion of food 

 material is at the vegetal pole. At this stage the egg cell, or, as 

 it is commonly termed, 'the yolk,' bursts forth from the ovary, is 

 taken into the ostium, and begins its passage to the exterior through 

 the oviduct. Normally, in the upper end of the oviduct, the egg 

 cell comes in contact with the sperm, and fertilization occurs, after 

 which cell division begins. The blastoderm of the Hen's egg is the 

 only part of the cell that divides. Thus we have partial, or mero- 

 blastic, cleavage in the Hen's egg as compared with total, or holo- 

 blastic, cleavage in the Frog's egg. Another type of meroblastic 

 cleavage was earlier noted in the Arthropod egg. (W. f. 169.) 



The fertilized egg continues its passage down the oviduct, and 

 the cells in the anterior glandular portion of the oviduct secrete 

 several layers of albumen, or 'white,' around it; a portion of 

 which, lying in contact with the vitelline membrane surrounding 

 the egg, form the chalazae. These make it possible for the 

 embryo to maintain a definite orientation during development. 

 Farther posterior other secreting cells form two thin, enclosing 

 membranes which lie just within the shell. Still farther down the 

 oviduct other secreting cells give off a material which hardens to 

 form the outer egg shell. The passage through the oviduct 

 usually takes 24 hours, so that if the egg is fertilized in the anterior 

 end of the oviduct, it has reached the 24-hour stage of development 

 at the time it is laid. When the egg is laid, development ceases 



