XIV 



ELEMENTS OF VERTEBRATE EMBRYOLOGY 



451 



by a flattening of the thick yolky portion of the blastula wall (A). The 

 flattened portion gradually becomes invaginated^ or in other words it 

 curves inwards (B, C, 



D), to form a cup- /^i'%^ A ^ssiSKX o 



shaped gastrula (E, 

 F). The wide mouth 

 of the gastrula 

 becomes gradually 

 reduced to a small 

 opening — the blasto- 

 pore (Fig. H) — the 

 portion of the gas- 

 trular rim which later 

 stages show to be an- 

 terior, or headwardj 

 in position gradually 

 extending backwards 

 so as to roof in the 

 gastrular cavity. The 

 blastopore marks the 

 hinder end of what 

 will be the dorsal sur- 

 face of the larva. 



At this stage there 

 are present only the 

 two primary layers 

 of cells — ectoderm 

 covering the outer 

 surface and endoderm 

 lining the gastrular 

 cavity or archenteron. 

 The mesoderm how- 

 ever now begins to 

 make its appearance 

 and the mode in which 

 it does so in Amphi- 

 oxus is of great im- 

 portance inasmuch as 

 it has in all prob- 

 ability departed less from the primitive method than is the case with 

 any other vertebrates and on that account gives us important clues to 



Fig. iS 



Gastrulation in Amphioxus. (From Graham Kerr's Embryology.) 

 The second polar body is still adherent to the surface of the egg 

 in the neighbourhood of the original apical pole. The individual 

 figures are viewed from what is seen later on to be the left side of 

 the Amphioxus, the dorsal side being above and the head end 

 pointing towards the left side of the page. 



