ORGANIC FORM AND ARCHITECTURE 



685 



\-'.y 



Fig. 98. 



Diagram of the Foetal Membrane in a Mammal. After Turner. E, ectoderm 

 of embryo; H, gut, lined with endoderm; UV, umbilical vesicle or yolklcss 

 yolk-sac; AC, amniotic cavity; am, amnion proper; ALC, allantoic 

 cavity, outgrowth from the posterior end of the gut or enteron ; al, inner 

 wall of the allantois; 3p, the early ectodermal envelope or trophoblast. 

 which gives off preliminary processes; z/j, which attach the embryo to the 

 wall of the uterus or womb; sz, the subzonal membrane, round the 

 whole embryo; N, the mesoderm of the embryo. 



Fig. 99. 

 The Hyoid Apparatus of a Turtle. From a specimen. BH, the body of the 

 hyoid (basihyal); H, anterior part of the hyoid arch; AC, anterior comu, 

 a transformed first branchial arch; PC, posterior comu. representing a 

 transformed second branchial arch. The figure illustrates that mode of 

 evolution which consists in transforming an old structure into a quite 

 new one of fresh significance. The use of the apparatus in the turtle U 

 to afford insertion for muscles. 



