I ill: ORGANS OF Till i \ \ KR OR MR8KNC11YMK. 553 



i':>r as regard - tl,.- purpose of the umbilical circulativ* 

 Mih.M-rve> in I; ion of respiration. Foi 



allantoi>. when it has become larger, in the Chick for example, 



applies it -t-lf cl. -ly in the serottu : the 



vicinity ..' the air-ehainhcr and und-Tii'-at 1. tin* shell, SO that the 

 Mood cin-ulaiini: in it can enter into an exchange of gates with the 

 atmospheric air. It loMI it- importance for re*pi ogg 



only at the moment \sli.-utli.- r|,i. k hcak Livak 



the surrounding emhryonie in.-inl.i-an.>. and hreathes di air 



;ined in tin- aii- chamher. \-\>r tin- OOodttioOl 

 are now ah' red tln-ou^lioui tli.- whole body, since with the I- 

 nin^r of the process <>i' i-.-|.'n-ai i.n tl..- lungs are in a coi*-- 

 up a uM.atri .[tiantityof blood, resulting in a degeneration oi 

 ui:iliilira! compare also p. 584). 



Tin- iituliil'n'iil cr J umal* (fig. 139 Af) 



|'lay> a still more import , for here the t\\o uml> 



artrri.-s >n\ey the blood to the placenta. After the blood has 

 1'tM-n ladi'n in this oriran with oxj'gen and nutritive substances, it 

 tlo\v> hack a^ain to tlie heart, at lirst through t\. 

 unihilical v. in (j>. 5S4). 



A'. // /'rf/ter Development of the Vascular System up to the 



(a) The Metamorphosis of th> to a Heart 



>ra. 



As has been >luwn in a pi. ceding set ' a Verte- 



originally has for a short time the form of a straight sac, v i 

 sends oil' at its anterior end the two . dies, whi 



i end the two omphalomesenteric veins. The 

 ies far forward immediately behind the head on the ventral side 

 of the neck (tig. ngation of the body-ca 



parietal or cervical cavity). It is here attached by means of a 

 mesentery of onh which stretches from the alimei. 



canal to the ventral wall of the throat d by 



the cardiac >ac itself into an upper [dorsal] and an under part, or 

 mesocardimu |Mtn-iu> and anU-i 



During the first period of finhr\ lopment the heart is 



distiiiiruishi'd h\ a vt-ry considerable growth, especially in the longi- 

 tudinal direction; consequently it soon ceases to li necessary 



