CHAPTER XXXI. 



THE FOUR TO SIX SOMITE STAGE 

 (About Twenty-four Hours) 



As the embryo is already well on its way in development at the 

 time the egg is laid, and as it has been shown that the extent of devel- 

 opment varies considerably on account of the retention of the egg in 

 the hen for an extra twelve to sixteen hours if it is not ready for laying 

 sufficiently early in the day, the formation of the block-like portions 

 of mesoderm the somites becomes the more accurate measurement 

 of the age of an embryo. Chicks with the same number of somites do 

 not usually vary much among themselves in general, though individual 

 parts often do ; while chicks having been incubated for the same number 

 of hours vary considerably in all parts. 



The twenty-four hour stage (four to six somites), (compare Figs. 

 264 and 266), is of great importance, for it is during this very early period 

 of the chick's life that the interesting and important differentiating 

 processes are noted. Up to the time the first four somites form the hind- 

 brain, the entire growth of the embryo from Hensen's node cephalad, 

 has been a formation of the head-region only. 



There has been some question in the past as to whether or not ad- 

 ditional somites are formed anterior to the first ones thus laid down. 

 Patterson performed an interesting experiment which seems to warrant 

 our saying that such is not the case. 



Professor Patterson incubated six eggs up to the one somite forma- 

 tion period, and then with the most asceptic precautions, opened the 

 eggs and marked the first somite by injuring it with an electric needle, 

 or inserted a minute glass pin therein. The shell was then again closed 

 by a small piece of egg-shell, and the eggs again incubated for varying 

 number of hours before being reopened. No new somites appeared an- 

 terior to the injured one. 



In the study of whole mounts under the microscope, it must be re- 

 membered that reflected light coming up from below the object shows 

 different densities as darker or lighter areas. Any portions of the em- 

 bryo which have become thickened or folded over will therefore appear 

 extremely dark and be thus distinguished from the thinner and lighter 

 areas. 



At the end of about twenty-four hours we then have : 



1. Three definite germ layers. (Fig. 271.) 



2. Four to five somites, forming in the vertebral plates, which verte- 



bral plates have separated from the lateral plates. 



3. The mesoderm divided into a somatic and a splanchnic layer. 



