810 



SIGURD V. BOLETZKY 



The junction of the branchial veins (formed at stage XIII) to the heart occurs 

 at stage XIII-XIV, long before the beginning of branchial vascularisation. 



The branchial hearts begin to pulsate at about the end of stage XII; the heart. 

 at the later stage XIII. 



The formation of the aortae and their junction to the venous System occurs 

 at the stages XIII and XIV, the posterior aorta taking over a part of the mantle 

 supply, the cephalic aorta that of the cephalic organs and of the arm rudiments 

 (Fig. 16, 17a). 



The development of the ganglia results in a new connexion between the ventral i | 

 parts of the cephalic sinus (at the stages XIV and XV), (Fig. 16, 18b, c). 



At the same time, the posterior sinus is restricted by development of the 

 pericardium, the mantle veins are separated from it and joined to the venae 

 cavae by newly-formed vessels (Fig. 18b, 19a). 



The définitive bifurcation of the vena cava first appears at stage XIV as a 

 posterior link between the two primary vena cava limbs and is completed by 

 the growth of the inner yolk sac at stage XV that leads to an elongation of the 

 hind-gut complex (Fig. 16b, 18b). 



This inner concentration of yolk is above ail responsible for the typical 

 shaping of the sinus mesentericus (Fig. 18a). 



The formation of the définitive arm-vein ring and its connexion with the 

 embryonic ring formed at stage XIV occurs at stage XV; and its dorsal closure 

 results from the union of the dorsal arms at stage XV-XVI, which is also respon- 

 sible for the formation of the buccal sinus (Fig. 18, 16b). 



The " white body " (the organ for blood-cell formation) is formed from a 

 membrane which appears at stage XIV-XV in the ophthalmic sinus, at the inner 

 side of the eyeball (Fig. 17a, d, e). 



Bit 



ZE1CHENERKLARUNG 



A.b. 



Arteriae buccales 



A.br.c. 



„ brachiales communes 



A.brn. 



Arteria branchialis 



A.coll. 



„ collaris 



A.hg. 



„ hepatogastrica 



A.o. 



„ ophthalmica 



A.p.l.s. 



pallialis lateralis sinistra 



A.p.l.d. 



dextra 



A.ped. 



Arteriae pédales 



A. p. m. 



Arteria pallialis média 



A.s.i. 



„ siphonalis inferior 



A. s. s. 



„ ,, superior 



Ao.c. 



Aorta cephalica 



Ao.p. 



„ posterior 



1 



