4i6 AN INTRODUCTION TO ZOOLOGY 



turn inwards to form the two ducti Cuvieri, which enter the sinus 

 venosus, one on each side. The posterior cardinal vein arises as an 

 outgrowth from the ductus Cuvieri and passes backwards laterally 

 to the somites and above the intermediate cell mass to the tail. 

 At a later stage the two lateral vitelline veins unite to form a single 

 vessel, the ductus venosus or main splanchnic vein, opening into the 

 posterior end of the sinus venosus. The two anterior vitelline 

 veins fuse near the sinus terminalis and the right vein then dis- 

 appears, leaving the left to open into the posterior end of the 

 heart. 



Delimitation of the Embryo, 



The manner in which the head end of the embryo grows off the 

 blastoderm as a head fold has already been discussed, and it was 

 noted that it produced a head fold bay, which may appropriately 

 be termed the anterior limiting sulcus, since it separates the anterior 

 end of the embryo from the blastoderm. This general embryonic 

 growth, early marked at the front end, goes on somewhat more 

 slowly over the whole embryo in length, breadth and thickness. 

 After the eighteenth pair of somites have been formed, the primitive 

 streak as such disappears, and is represented only as an area of active 

 cell proliferation, the tail bud. By the time twenty-six somite pairs 

 are established, i.e. about 46 hours, this point grows off the blasto- 

 derm in a similar manner to the head, thus bringing into being a 

 tail fold and a posterior limiting sulcus. This growth involves also 

 the entoderm, giving rise to a hind gut. The intervening part of the 

 embryo, too, has been getting thicker, and so rising above the 

 blastoderm, and a little later, as this upstanding part widens, we 

 have produced the lateral limiting sulci, connecting up the anterior 

 and posterior sulci. In this manner, then, the actual embryo becomes 

 sharply marked off from the surrounding blastoderm by an encircling 

 groove. 



Thus, from being just a thickening in the middle region of 

 the blastoderm, and passing out insensibly into the extra-embryonal 

 region, the embryo becomes a sharply delimited structure. Even 

 before this marking off is complete general changes have been taking 

 place in the conformation of the embryo as a whole that need to be 

 considered before passing on. Up to the stage where twleve pairs 

 of somites are present (about 33 hours) the axis of the embryo is 

 fairly straight, but when 15 somites have appeared (about 36 hours) 

 it will be noted that the head end is turning definitely to the right ; 

 the bend, or cranial flexure, is brought about by the rapid growth 

 of the roof of the mid-brain. The turning is not due to the left side 

 growing faster than the right, but to the combined twisting and 



