TK.* \S\ ERSE SECTIONS OF EMBRYO OF 12 MM. 261 



The accompanying *f.ure 188 represents the outline of the pig embryo which 

 was cut into the series of transverse sections from which figures 189 to 198 have 

 been made. The student can easily identify the parts in the figure by comparison 

 with that of the pig of 10 mm. (Fig. j66), aided by the accompanying description 

 of the same. The sections of this embryo are lo/n in thickness, and are 966 

 in number, not 1200, as the student might expect. The discrepancy is due 

 to the shrinkage of the embryo when imbedded in paraffin. The shrinkage is 

 always very great, and in the case of embryos causes a loss of almost 20 per cent 

 in the length; but as it seems to take place uniformly throughout the embryo, it 

 causes no distortion, so that the embryo in paraffin is an exact though greatly 

 reduced copy so to speak of the living embryo. It should be remembered that 

 no correct measurements of the size of organs or cells can be obtained from sec- 

 tions made by the paraffin method. This limitation upon the use of sections 

 too often forgotten. The horizontal lines indicate approximately the levels at whic 

 the sections here 7 figured belong. For convenience the direction and position of the 

 frontal sections represented in figures 203 to 207 are also indicated approximately 

 on the same outline, although, of course, the frontal series was from another 

 embryo. 



Pig Embryo of 12.0 mm. Study of Transverse Sections. 



The figures and descriptions here presented of ten sections have been 

 selected as illustrating the most important structures, with the exception of the 

 umbilical opening and of the kidney, which can be better represented in sections 

 from older or younger stages. 



Section through the I * pp> 



by the line 185, this sectiQffcJ ^ taken from a level about 



and the apex of the otocyst, Ot. It passes, therefore, through the fore-brain, 

 and the fourth ventricle, Vcn.IV, or cavity of the hind-brain. The section^H 

 bounded by a thin layer of cpjfiermis, between which and the brain-wall there is 

 a large amount of mesenc hymal tissue. Alongside the hind-brain lies a series of 

 important structures imbe \<\d in the mesenchyma, which are identical upon the 



ciH<i. althoi. Bter somewhat' in the section, as the plane of cutting 



was not symm< rse for the head. These structures are in the fol- 



lowing order: .V.; rigeminal ganglior. , A T ./,8, the acustico-facial ganglion 



complex; Ot, the otocyst, an oval vesicle with very distinct epithelial walls. Next 

 the ninth or glosso-pharyngeal nerve (scarcely appearing in the section on the right 

 side of the embryo i is shown by the upper ^portion of its ganglion on the ' 

 (the right in t v vi; tb- 



close to the me'*' 

 posteri< 

 thr 



