332 PROFESSORS ARTHUR ROBINSON AND A. GIBSON. 



The material given to Mr Gibson, for reconstruction, was a series of 387 trans- 

 verse sections, each section being IOju, thick ; therefore the total length of the 

 fixed and hardened embryo was 3'87 mm. Shrinkage, to the extent of about 

 40 per cent., must, therefore, have occurred during the preparation and embedding 

 of the embryo. 



That shrinkage has occurred is obvious from the condition of the heart and some 

 of the other organs. Moreover, whilst the shrinkage does not appear to have altered 

 the relative positions of the organs, it has altered the relative proportions of the 

 two parts of the bent embryo ; for whilst in the fresh embryo the total length wae 

 11 mm., and the proportion of the shorter to the longer part was as 325 to 625, the 

 total length of the reconstruction is 819 mm. and the proportion of the shorter to 

 the longer segment is as 230 to 387. Nevertheless, the general appearance of the 

 reconstruction and the drawings of the fresh embryo are essentially alike (figs. 54, 

 55, 56, PI. XIX; 8, 11, PI. IX ; 27, PI. XVI), except that the flexure in the model 

 is more acute, on account of a certain amount of shrinkage of the heart, and that the 

 curve of the head and the part of the body on the cranial side of the umbilicus is less 

 rounded than it was in the fresh embryo. 



The reconstruction was made by the ordinary wax-plate method, each section 

 being enlarged 100 diameters on plates 1 mm. thick; the reconstruction is, therefore, 

 one hundred times as large as the fixed and hardened embryo. 



When the embryo was cut into sections, portions of the yolk-sac were still attached 

 to it, and they are shown in the model (Sp., figs. 54, 55, 56, PL XIX). 



There was a completely closed amnion, which was removed from the model to 

 expose the dorsal surface of the embryo. 



The description of the appearance of the embryo and its appendages in the 

 fresh condition has been given by Professor Ewart in the preceding pages ; what 

 follows is a description based upon the reconstruction. In it the terms " cranial " and 

 " caudal " portions of the embryo will be used respectively to refer to the shorter and 

 more cranial, and the longer and more caudal parts of the bent embryo ; whilst the 

 term " the bend of the embryo " will refer to that part which connects the cranial and 

 caudal portions together and forms the convex projection of the hook-shaped bend. 



Surface Views. 



AVhen examined from above, from which aspect the cranial portion of the embryo 

 is not visible, the caudal portion has an hour-glass-shaped outline (fig. 54, PI. XIX), 

 the constricted portion being a little nearer the caudal than the cranial end of the 

 caudal portion of the embryo. 



The constriction is not an artefact, but it is exaggerated by the shrinkage, for it 

 is more marked in the reconstruction than in the fresh specimen (fig. 10, PI. XIII; 

 fig. 54, PL XIX). 



In the region of the constriction the somatic wall of the embryo is definitely 



