64 Journal of the Mitchell Society [September 



son cleavage .stajjes are very likely to be found, altliongh the major- 

 ity of eg?!? will be in the blastopore stage. This condition makes the 

 turtle an excellent animal from -which to secure the blastopore stages 

 of the discoidal cleavage type for class -work, since a single turtle 

 may produce six or more eggs in this stage, whereas the hen, which 

 must also be killed in order to secure the blastopore stage, will produce 

 but a single egg in this condition.* The use of turtle eggs permits 

 of a comparative study of the development of eggs of a clutch which 

 have, in the very nature of things, much more in common than any 

 group of hen eggs. Furthermore, turtle eggs may be placed under 

 identical incubation conditions immediately upon removal from the 

 oviduct, while the hen eggs, which are laid from day to day, are 

 subject to varying external conditions before incubation. 



For incubation, the eggs of Chrysemys cinerea are buried to a 

 depth of about one inch in horizontal position in a battery jar of 

 moist sand. Each group is labeled with a peg bearing the number 

 of the turtle. The embryos are numbered as they are removed from 

 the eggs and fixed. 



A few precautions are necessary in the artificial incubation of 

 C. cinerea eggs. In the first place, if eggs which have been laid 

 are to be incubated, they must be secured early after deposition, since 

 in from twenty-four to thirty-six hours after laying the embryonic 

 disc comes to lie just above the yolk level, either at the top or the 

 side or at the end of the egg. and attaches itself to the shell mem- 

 brane. Any change in the position of the egg. even as little as a 

 quarter of a turn, after this, may cause the yolk to settle over the 

 embryo and the devolopnuMit will cease, or become abnormal. This 

 probably accounts for the failure of many attempts at artificial in- 

 cubation where the eggs are brought in from the field several hours 

 after deposition. 



A proper moisture content also is essential for success. Eggs of 

 the painted turtle exposed to the air of laboratories soon become 

 wrinkled and finally collapse. In the case of the snapping turtle, 

 the .shell is much tougher and the shriveling does not take place but 

 the moisture leaves the egg and the yolk becomes a hardened mass 

 within the shell. Seven "snapper" eggs were completely dried at 



* Rpront work hy Riddle (1921) would indicnto that bliistoj ore stages niny be srrured 

 from live hp.nn hy intruiTiuseuliir injertions. 



