WEIGHT, SIZE, AND AGE OF THE HUMAN EMBRYO. 



149 



it is not possible to so control the caliper points that they come into perfect contact 

 with the ends of the embryo without indenting it, or without incurring the risk of 

 injuring the specimen. The difficulties are increased by the fact that the measure- 

 ment has to be carried on under fluid. To meet these conditions resort may be 

 had to one of the simpler types of measuring microscopes, such as are used for 

 calibrating thermometer scales or for measuring spectrum photographs. For our 

 own use I have found that a Leitz-Edinger brain microscope, remodeled as shown 

 in figure 2, answers the purpose very satisfactorily. The large glass platform 



with which it was originally 

 equipped was removed and 

 in its stead was substituted 

 a brass plate carrying a re- 

 volving table on which to 

 place a glass dish containing 

 the embryo to be measured. 

 This made possible the move- 

 ment of the embryo, so that 

 it could be brought to any 

 desired position under the 

 observing tube. The ob- 

 serving tube was equipped 

 with a 70 mm. objective and 

 a Leitz No. eyepiece hav- 

 ing a hair-line. Through 

 this optical system, with 

 the tube drawn out, it was 

 possible to see in sharp focus 

 an entire 12 mm. embryo 

 in a single field under a 

 magnification of about 10 

 diameters. A Vernier scale, 

 reading to 0.05 mm., was 

 attached to the support car- 

 rying the observing tube, by 

 means of which the excur- 

 sion of the tube to the right 

 or left could be determined. 

 This completed the apparatus. Its use is extremely simple. The embryo is brought 

 to rest under the tube with the axis selected for measurement placed parallel with the 

 frame on which the support of the tube rides. By turning the crank handle of the 

 threaded rod, which moves the observation tube from side to side, the tube is adj usted 

 so that the hair-line of the eyepiece is vertically sighted at one end of the embryo, as 

 with a surveyor's theodolite, and a reading is taken on the Vernier scale. The observa- 

 tion tube is then moved to the other end of the embryo and another vertical reading 

 is taken. The difference between the two readings gives the distance the tube has 



Fig. 2. — Instrument for measuring embryos. A', Leitz No. eyepiece with 

 cross-hairs; Z, 70 mm. Zeiss objective; Y, glass container for 

 embryo; V, standard on top of which the stationary plate of 

 the Vernier scale is mounted. The reading is made from the 

 rear of the instrument. 



