18 



meseraic veins." Thus the fact is undeniable, that the heart is developed 

 in the venous system in connection with the vitettus. The rhythmical ex- 

 pansions and contractions in this organ (the punctum saliens of early authors) 

 serve to pump the vitelline fluids in the bulbus arteriosus and the two aortse 

 which are developing at the other end of the heart. But for this air-ciishion 

 within the egg (Figs. 8 and 9, g), neither these rhythmical expansions in 

 the heart nor the changes in pressure for compelling circulation in the vitelline 

 fluids could take place, since the unyielding shell would inhibit these actions, as 

 must appear obvious. 



Respiration is provided for in the following manner : The aUantois (a diverticu- 

 lum of the intestinal canal) is pushed out around the amnion which contains the 

 embryo, and expanding its vast capillary net-work of vessels (whose footstalks 

 spring from the two iliac arteries, as do the umbilical arteries in the mammalian 

 embryo) against the shell-membrane or chorion becomes the respiratory organ of 

 the chick, by means of which the venous blood is constantly arterialized, the oxygen 

 passing in and the carbonic acid passing out through the pores in the shell by 

 the action of the polar forces. But " at the time the heart first begins to beat, 

 the capillary system of the vascular and pellucid areas is not yet completed, 

 and the fluid which is at first driven by the heart contains, according to most 



observers, very few corpuscles The course of the blood then, 



during the latter half of the second day, may be described as follows : The 

 blood brought by the omphalo-meseraic veins falls into the twisted cavity of 

 the heart, and is driven thence through the bulbus arteriosus and aortic 

 arches into the aortic trunk. From the aorta by far the greater part of the 

 blood flows into the omphalo-meseraic arteries, only a small amount passing on 

 into the caudal terminations. From the capillary net-work of the vascular and 

 pellucid area into which the omphalo-meseraic arteries discharge their contents, 

 part of the blood is gathered tip at once into the lateral or direct trunks of the 

 omphalo-meseraic veins. Part, however, goes into the middle region of each 

 lateral half of the sinus terminalis, and there divides on each side into two 

 streams. One stream, and that the larger one, flows in a forward direc- 

 tion until it reaches the point opposite the head, thence it returns by the 

 veins spoken of above, straight to the omphalo-meseraic trunks. The other 

 stream flows backward, and becomes lost at the point opposite the tail."* 



The following from the same authors is deeply suggestive: "Soon after its 

 formation the heart begins to beat, at first slow and rare pulsations, beginning 

 at the venous and passing on to the arterial end. It is of some interest to note 

 that its functional activity commences long before the cells of which it is com- 

 posed shoiv any distinct differentiation into muscular or nervous elements." It 

 would be difficult to overestimate this circumstance, since it establishes beyond 

 peradventure the power in the higher as in lower animals to effect rhythmical 

 expansions and contractions in the soft tissues in the absence of any muscle or 

 nerve for producing them. But at present the significance of these rhythmical 

 expansions and contractions taking place in the heart concerns us most, since 

 the manifest purpose is to increase circulation ; and as this can only be done by 



* Fos'.er and Balfour's ' Embryology." 



