206 



EMBRYOLOGICAL TYPES 



the latter and the endoderm which represents the foregut- 

 region of the enteron. As yet there is no floor to the gut, 

 nor is the ventral side of the embryo formed at all. The 

 mesoderm, lying on each side of the notochord becomes 

 segmented into somites. That part which is nearest to the 

 notochord will produce the myotomes ; farther laterally, a 

 split arises in the mesoderm which becomes the ccelomic 

 cavity, and which separates a somatic layer of mesoderm closely 

 applied to the ectoderm from a splanchnic layer which is 

 similarly applied to the endoderm. The ectoderm, mesoderm, 

 and endoderm extends to the side far beyond the limits of the 



Fig. 94. — Gallus : longitudinal section through the head-region of an 

 embryo chick after 30 hours' incubation. (From Jenkinson.) 



aip, anterior intestinal portal ; en, endoderm ; pc, pericardium ; pr, 

 proamnion ; spc, spinal cord ; st, stomodaeum. Other letters as Figs. 90, 

 91, and 93. The lines marked 1 to 5 indicate the planes of the transverse 

 sections shown in Figs. 95 and 96. 



embryo, and so it comes about that the ccelomic cavity of the 

 embryo is perfectly continuous with the " extra-embryonic " 

 ccelom. As this extra-embryonic splanchnic mesoderm 

 spreads out, blood-islands develop between it and the endo- 

 derm. This is seen in blastoderms observed by transparency 

 as the spreading of an area vasculosa over the area pellucida. 

 Eventually this area vasculosa spreads over most of the 

 blastoderm up to the germ-wall, except for a region immediately 

 in front of the head-fold which is known as the proamnion. 

 The peripheral extent of the area vasculosa is marked by a 

 blood-vessel, the sinus terminalis. 



Nerve-tube. — The neural plate develops as a thickening 



