Formation of Coelomic Cavity 



27 



there should not be a blastopore during the time of the formation 

 of the archenteron. If there were a blastopore at this time, clearly 

 there could be no cavity produced by swelling. 



The amnion is essentially an organ of the coelom. This view has 

 been taken by Hubrecht in his paper upon the " Early development 

 of the Hedgehog and its significance for Phylogeny of the Amnion." 



But it has also probably arisen in correlation with a distended 

 archenteron. 



In Elasmobranchs there is a very heavily yolked egg, but there 

 is no amnion. Nevertheless it is very interesting to note in the 

 Elasmobranchs a condition which might very easily lead to the 



am 



A B 



Fig. 14. A, Scyllium; B, Amniote. 



Diagrammatic section. Black = endoderm ; dotted line = mesoderm ; line barred 

 with white = ectoderm ; coe coelom ; am amnion ; arch archenteron. 



formation of an amnion of the amniotan type. In Dogfish 

 embryos of about half an inch in length, at a time just after 

 the closure of the blastopore, there will be found two large 

 coelomic cavities upon the yolk sac, that is to say cavities of the 

 so-called exembryonic coelom, which are morphologically and 

 physiologically in the position occupied by the lateral folds of the 

 amnion of an amniote (Fig. 14). 



In Scyllium there is no accumulation of fluid within the gut 

 cavity, and the rather long yolk stalk keeps the so-called embryo 

 part well away from the yolk sac. 



But a very slight alteration of conditions could convert this 

 into an amniote as Fig. 14 illustrates. 



