18 EMBRYOLOGY. 



1-5% fat and other extractive materials, 0'5% salts (potassic chloride, 

 sodic chloride, sulphates, and phosphates), and 86% water. It 

 surrounds the yolk in several layers of varying consistency. There 

 is a layer quite closely investing the latter, which is firmer and 

 especially noteworthy because it is prolonged into two peculiar 

 spirally twisted cords, the chalazce (ch.l), which consist of a very 

 compact albuminous substance, and which make their way through 

 the albumen to the blunt and to the pointed poles of the egg 



The albumen is enclosed by the thin but firm shell-membrane (s.m) 

 (membrana testae), which is composed of felted fibres. It may be 

 separated into two lamellae an outer, which is thicker and firmer, 

 and an inner, which is thinner and smooth. Soon after the egg is 

 laid the two layers separate from each other at the blunt pole, and 

 enclose between them a space filled with air (a.ch), the so-called 

 air-chamber, which continues to increase in size during incubation, 

 and is of importance for the respiration of the developing Chick. 



Finally, the shell, or testa (a), is in close contact with the shell- 

 membrane; it consists of an organic matrix (2%), in which 98% cal- 

 careous salts are deposited. It is porous, being traversed by small 

 canals, through which the atmospheric air may gain entrance to the 

 egg. The porosity of the calcareous shell is an absolute necessity for 

 the normal development of the egg, since the vital processes in the 

 protoplasm can take place only when there is a constant supply of 

 oxygen. If the porosity of the shell be destroyed, either by soaking 

 it in oil or closing its pores with varnish, the death of the incubated 

 egg ensues in a very short time. 



B. Compound Eggs. 



Compound eggs are found only in a few subdivisions of the 

 invertebrated animals, as in the Cestodes, Trematodes, etc. ; they 

 are noteworthy in this respect, that they are produced by the 

 union of numerous cells, which are formed in two different glands 

 of the sexual apparatus of the female, in the germarium and in 

 the vitellarium. In the germarium is developed the egg-cell in the 

 restricted sense. This is always very small, and consists almost 

 exclusively of egg-plasm. When this cell at its maturity is set free 

 from its surroundings and cornea into the sexual outlets, it is obliged 

 to pass the opening of the vitellarium ; here there are associated 

 with it a number of yolk-cells, which, owing to deposition of reserve 

 material in the protoplasm, appear turbid and coarsely granular, 



