4i8 AN INTRODUCTION TO ZOOLOGY 



head turns uppermost and the left side downwards. It is probably 

 a mechanical result of the cranial flexure, for the head is unable to 

 bend straight downwards, owing to the presence of the yolk. At the 

 stage of 20 somites (about 43 hours) this is very clearly marked. 

 Five hours later (27 somites) another bend has made its appearance, 

 this is the cervical flexure, and is produced by the increased growth 

 of the roof of the hind-brain, causing a curving of the whole hind- 

 brain region. The result of these two flexures and the enlargement 

 of the fore-brain vesicle is that the front end of the head becomes 

 directed backwards and finally inwards also. The maximum 

 amount of bending is reached in the stage of 35 pairs of somites 

 (about 72 hours). The appearance of the actively growing tail-bud 

 leads to a similar but much less extensive bending of the small 

 posterior end of the embryo. This also turns to the right, and is 

 well marked at 72 hours. Indeed, the whole embryo tends to turn 

 in the same direction, save in the region of the vitelline veins, so 

 that it comes to lie nearly on its left side, and the heart also becomes 

 displaced to the right. 



Foetal Membranes. 



The foetal membranes are four in number, viz. : the amnion, 

 the chorion, the yolk-sac and the allantois. The first three of these 

 are produced by the extra-embryonal blastoderm, while the last is 

 an actual outgrowth from the body of the embryo itself. 



The first two are developed together as the result of one process, 

 and we may commence with them. The actual details of their 

 formation are somewhat complicated, but it will suffice if only the 

 general outlines are considered. Their formation is initiated in a 

 chick about 30 hours old by the appearance of a transverse ridge 

 across the pro-amniotic area in front of the head. This increases 

 in height, and a section across it shows that originally it involves the 

 ectoderm and the entoderm ; at a later stage, however, the mesoderm 

 and ccelom invade this area, and so the fold ultimately involves only 

 the ectoderm and the somatic mesoderm, while the entoderm, with its 

 accompanying splanchnic mesoderm, sinks back again to a lower 

 level. The head fold of the amnion, as this ridge is termed, assumes 

 a crescentic shape, and its ends pass back as the lateral amniotic 

 folds. The fold gets higher and grows back fairly rapidly, com- 

 pletely enclosing the head, and at 48 hours (26-28 somites) has 

 reached back almost to the level of the vitelline veins. At this time 

 a tail fold of the amnion appears, similar to the anterior one, save 

 that it consists of ectoderm and somatic mesoderm from the very 

 commencement, and never involves the entoderm. Its lateral folds 

 join up with the pre-existing ones, so that the whole embryo is 



