352 CLASS xvi. 



the albumen is first diminished by evaporation, by which the air- 

 space at the obtuse end is enlarged ; the yolk changed into a milk- 

 coloured fluid becomes surrounded (on the sixth or seventh day) 

 by the constantly growing germinal membrane and forms a yolk- 

 sac connected with the embryo; finally (on the nineteenth or 

 twentieth day) this yolk-sac is taken into the abdominal cavity of 

 the embryo. 



The first commencement of development consists in this, that 

 the germ (a round disciform lamina lying immediately under the 

 yolk-membrane) separates itself more and more from the other parts 

 of the egg. The germ becomes firmer and more membranous; the 

 middle becomes transparent (area pellueida s. germinativa], and 

 has a darker margin; at the same time two or three concentric rings 

 (halones) in the yolk appear around the germ. The clear area has 

 first a round, then an oval form, but as early as the second day 

 becomes elongated, round and broad at both extremities and narrow 

 in the middle. In the germ itself two layers or laminae are 

 developed ; the uppermost layer is the serous or animal layer, the 

 lowest the mucous or vegetative. The serous layer consists of 

 round or multangular flat cells, with a nucleus and small granules, 

 placed close together; the mucous layer consists of cells without 

 nucleus which are smaller in the clear area, whilst at the margins 

 of this layer larger and smaller cells are intermingled with darker 

 globules and small granules lying diffused in a formless intercel- 

 lular substance 1 . Somewhat later a third layer is seen between 

 the other two which is named the vascular layer (see above, p. 5). 



As the germ separates itself from the other parts of the egg, so 

 also a separation takes place in the germ itself; one part of it 

 becomes the embryo whilst the other retains the name of germinal 

 membrane into which the embryo passes without any determi- 

 nate boundary at first. Afterwards the two layers of the germ 

 exist in the embryo, and in it the dorsal part is first formed 

 from the serous layer. The first rudiment of the chick is seen 

 about the fourteenth or fifteenth hour of brooding as a narrow 

 streak (nota primitiva] which is somewhat broader at one extremity 

 and about 1^ line long; this streak lies in the long axis of the 

 transparent field (area pellueida} , which itself is situated not in the 



1 SCHWANN Mikrosk&p. Untersuchungen, a. 65 67, Tab. n. figs. 6, 7. 



