114 THE EGGS OF MAMMALS 



ent differentiation as seen in whole mount preparations. 

 Localized thickenings only occur. 



(b) The stage of posterior thickening and initial elongation 

 of the embryonic disc develops one or two beating hearts, 

 and localized thickenings after 4-5 days' growth in vitro. 



(c) The stage of short primitive streak undergoes marked 

 elongation of the primitive streak and embryonic disc on the 

 2nd day; two, and in one case three, beating hearts appeared 

 after 2-3 days of culture. 



(d) The stage of medium primitive streak gives results 

 comparable to (c) . In several instances brain, hearts, neural 

 tube and somites appear. 



(e) The stage of long primitive streak gave rise to em- 

 bryos with as many as six pairs of somites after 1 day of 

 culture, and the pre-somite and two-somite stages give only 

 slightly, if at all, better development. 



Nicholas and Rudnick (1934) similarly were unable to 

 obtain any adequate development of rat blastocysts in stages 

 earlier than the pre-somite or 5-7 somite. But vesicles in the 

 latter stages developed markedly in a medium consisting of 

 equal parts of rat plasma and 14-15 day rat embryo extract. 

 They report that growth occurs during the first twenty-four 

 hours in vitro gradually slowing and ceasing by the 36th hour. 

 ''At 48 hours or earlier, differentiation in the embryo has 

 reached a maximum, at which it may be maintained for 

 another 24 hours. 



''During this period the embryos in the best cases have 

 differentiated from 2 to 16 somites. The allantoic bud has 

 grown from a small lump of tissue at the angle between the 

 amnion and the posterior part of the embryo to join with the 

 superior surface of the ectoplacental cone. The heart, un- 

 formed at the time of implantation, has differentiated a two 

 chambered structure and has initiated its beat, the blood 

 islands have developed in the yolk sac epithelium, and cir- 

 culation has commenced, both in the yolk sac and in the 

 embryo. The nervous system has differentiated consider- 

 ably; eyes and ears have differentiated and the embryo as 



