TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. ■ 175 



name correctly all the parts shown in each section and to identify the distribution 

 of the three germ-layers in every case.* A sufficient number of high-power 

 drawings ought to be added to illustrate the character of the various tissues. 



The accompanying figure 112 represents the outline of the pig embryo which 

 was cut into the series of transverse sections from which figures 1 1 3 to 1 2 1 have 

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

 with that of the pig of 10 mm. (Fig. 99), aided by the accompanying description 

 of the same. The sections of this embryo are ten mikrons 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 exatt 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 

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

 is too often forgotten. The horizontal lines indicate approximately the levels 

 at which the sections here figured belong. For convenience the direction and 

 position of the frontal sections represented in figures 1 22 to 1 25 are also indicated 

 approximately on this same outline, although, of course, the frontal series was 

 from another embryo. 



The Study of Transverse Sections. 



The figures and descriptions here presented of eight sections have been 

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

 disposition of the umbilical opening and of the kidney, which can be better repre- 

 sented in sections from older or younger stages. 



Section through the Upper Part of the Otocyst. — As shown in figure 1 12, by the 

 line 185, this section is taken from a level about half-way between the eye and 

 the highest point in figure 112. It passes, therefore, through the fore-brain, 

 F. b, and the fourth ventricle, Ven. IV, or cavity of the hind-brain. The section 

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

 is a large amount of mesenchymal tissue. Alongside the hind-brain lies a series 

 of important structures imbedded in the mesenchyma, which are identical upon 

 the two sides, although they differ somewhat in the section, as the plane of cut- 

 ting was not symmetrically transverse for the head. These structures are in the 



* For making camera lucida drawings, a 2-inch objective will be found convenient. An Abbe camera 

 is recommended. Compare the directions for drawing in Chapter VIII. 



