REPRODUCTION AND DEVELOPMENT 59 



stand what stage of development has probably been reached. Draw 

 (XI) the egg as thus dissected. 



Exercise 69. — The Twenty-four-hour Chick Embryo. 



(d) Open an egg which has been incubated for twenty-four hours 

 and, placing it on the cotton beside the one just drawn, compare the 

 two. Record or make a simple sketch to show the changes which have 

 taken place in the blastoderm during this first day of incubation. 

 Before discarding this specimen, the existence of a delicate egg- 

 membrane should be demonstrated by puncturing the yolk. 



(e) Permanently mounted specimens of the unincubated blastoderm 

 and the developing embryo may be used for the study of approximately 

 the twenty-four-, thirty-six-, and forty-eight-hour stages, in their finer 

 details. These should be handled with great care lest they be crushed 

 by wiping and are to be studied with the handlens or the low-power 

 objective of the microscope; do not use the high-power objective. 

 Such slides are secured by removing the blastoderms, which are then 

 killed, stained, and mounted in balsam. The first to be studied is the 

 twenty-four-hour stage in which the following parts are to be made 

 out: neural folds, notochord, mesodermal somites, primitive streak, 

 area pellucida, and vascular area. Focus carefully to determine the 

 vertical relationships of the parts and compare your results with what 

 is shown by diagrams. Draw this stage as a full-page figure. 



Exercise 70. — The Thirty-six-hour Chick Embryo. 



(f) Open an egg which has been incubated for thirty-six hours and 

 notice the changes which have occurred. With the aid of an instructor, 

 inject some India ink into the cavity beneath the blastoderm and then 

 harden the embryo by dropping strong alcohol upon the outside of 

 the blastoderm. Compare, part by part, with a permanently mounted 

 specimen of the same stage, placing the latter against a white back- 

 ground. Study with handlens and low-power objective the parts pre- 

 viously observed in the twenty-four-hour stage. Observe the mounted 

 specimen further under the compound microscope and make out, also, 

 the beginning of the brain vesicles, the optic vesicles, the anterior 

 amniotic fold, the omphalo-mesenteric veins, the heart, and any 

 changes in the size and proportions of parts. Here again, careful focus- 

 ing and the comparison of what you see with models and diagrams are 

 desirable for the proper understanding of the third dimension. Draw 

 this stage in a figure similar to the last. 



