124 DEVELOPMENT OF THE CHICKS 



Exercise XXIV 



in a labeled sketch. Your partner will at the 

 same time be examining an embryo at another 

 stage, so that each pair of students will have a 

 more-or-less complete picture of early develop- 

 ment. Each student also will perform a test for 

 cytochrome oxidase on his chick embryo. 



Prepared slides of chick embryos will be avail- 

 able for examination under the microscope, 

 representing stages of development both earlier 

 and later than your live embryos. Examine 

 them carefully, tracing the development of 

 various parts of the embryo from stage to 

 stage: the heart, brain, eye, limbs, musculature, 

 and so on. 



EARLY STAGES OF THE 

 CHICK EMBRYO 



Two students will work together on this 

 experiment. Each pair will be given two eggs, 

 one of which has been incubated at 38°C for 3 

 days and one for 5 days. Each egg is marked 

 with the number of days of incubation. 



In an egg left resting in one position for any 

 length of time, the embryo has rotated so that 

 the blastodisc is at the top, owing to the yolk 

 being heavier. You cannot rely on this in the 

 eggs given to you, and you should "candle" 

 your egg to find where the embryo lies. This is 

 conveniently done by holding the egg in front 

 of a microscope lamp. Mark with a pencil the 

 place on the egg where a shadow shows the 

 embryo to lie, and keep this uppermost. Lay 

 the egg in a petri dish, partly filled with warm 

 Ringer solution, and carefully cut around the 

 middle of the shell with scissors. Pick the shell 

 off carefully. The unbroken yolk and embryo 

 then will lie free in the Ringer solution. If a 

 living embryo is not present, get another egg 

 from the instructor. Use a dissecting microscope 

 for observing the embryo. 



Up to gastrulation, the chick follows much 

 the same pattern of development as does the 

 echinoderm egg. The differences are due mainly 

 to the large amount of yolk in the chick egg, 

 which crowds the protoplasm of the egg into a 



flat disc. Development proceeds primarily in 

 this disc, rather than in the whole sphere of the 

 egg as in a sea urchin. The embryonic heart 

 begins to beat after 2 days, and some circula- 

 tion of blood may be detected then. Anterior 

 to the heart is the head, with its bulging, partly 

 formed eyes. After 2 days the optic vesicles 

 reach the optic cup stage and lenses form. Also 

 the somites, precursors of the muscles, appear 

 at about the same time, as blocks of tissue lined 

 up in two rows along the trunk of the embryo. 

 The vitelline blood vessels which carry food to 

 the embryo emerge from the middle of the trunk 

 and branch out over the yolk. 



Three-day embryo. The embryo is bent back 

 on itself, and lying on its side. Note the size of 

 the head and the development of the heart. 

 Two limb buds should be visible on each side 

 of the embryo, as projecting lumps of tissue. 

 The anterior limb buds will give rise to the 

 wings, the posterior limb buds to the legs. 

 Count the number of somites and note the 

 development of the blood vessels surrounding 

 the embryo. Record your observations in a 

 labeled sketch. 



Five-day embryo. The increased size and 

 vascularity at this stage are obvious. It may 

 help make fine details visible to rinse the embryo 

 with several changes of warm Ringer solution. 

 Blood vessels can be seen pressed close against 

 the shell. What is their function? Toward the 

 tail end of the embryo you should be able to see 

 a fluid-filled sac, the allantois, which functions 

 as a urinary bladder, and is one of the extra- 

 embryonic membranes. As the embryo metabo- 

 lizes the food material of the yolk, waste prod- 

 ucts accumulate in this sac. The embryo is sur- 

 rounded by the amnion, another extraembryonic 

 membrane, but this is difficult to see after the 

 egg has been opened. 



CYTOCHROME OXIDASE IN THE 

 CHICK EMBRYO 



One can detect the presence of cytochrome 

 oxidase in tissues with the so-called NADI 



