OKLAHOMA ACADEMY OF SCIENCE 47 



plication with the mesoderm and entoderm following in the order 

 given. A large part of the ectoderm is made up of brain and 

 spinal cord tissue. These columnar cells are tightly pressed 

 together and show a high rate of growth while the 

 rounder mesenchyme cells which make up a large part 

 of the mesoderm are loosely packed and show a slower rate. In 

 the 18-20 hour stage the mesoderm slightly exceeded the ectoderm 

 but this was probably due to the fact that in this stage the meso- 

 derm was just forming. This high rate of growth of the ectoderm, 

 the larger part of which is nervous tissue proves that a higher per- 

 centage of cell division takes place in the central dorsal region 

 of the chick than in the lateral ventral parts. 



In the early stages of the chick preceeding the differentiation 

 of organs cell division takes place everywhere throughout the tissue, 

 although in places a higher percentage is observed than in others. 

 As development progresses in the later stages definite centers of 

 growth appear. In these centers the cells continue to multiply while 

 in the immediately surrounding parts they cease. Numerous ex- 

 amples of these foci of growth may be cited. For instance, in the 

 brain and spinal cord tissue all the dividing cells are found toward 

 the inside of the tube. These dividing cells are small and round 

 while the surrounding cells which do not multiply are much longer 

 and larger. The dividing nuclei in the oesophagus, stomach, intes- 

 tine, trachea, etc., are also along the inside of the tube. In the 

 48 hour chick the walls of the lens sac are practically of even thick- 

 ness whh mitotic figures scattered along anywhere in the region 

 toward the cavity, v/hile in the 72 hour stage the inner layer has 

 greatly thickened and the cavity is almost obliterated. Few mitotic 

 figures can be seen in this thickened and uneven layer, although in 

 the outer layers, which are still undifferentiated, numerous mitotic 

 figures are scattered throughout. Various other examples can be 

 given but these are sufficient to show that in the earlier stages the 

 growth is somewhat diffuse but as development progresses it be- 

 comes more focal. 



Summarizing ,1 find that the highest rate of grow^th is shown 

 in the 18-20 hour stage, next highest in the 48, with 33 and 72 hour 

 stages following in their respective order. As a result of this 

 study one sees that young cells multiply freely and in consequence 

 grow rapidly. When they are older they loose this capacity and 

 their growth is correspondingly lessened. In general, quoting Minot, 

 "We note that wherever a trace of differentiation has occurred 

 the rate of growth is diminished, where that differentiation does 

 show itself, the rate of growth may even increase in order to 



