THIRD PERIOD OF DEVELOPMENT. 



In this period of development we would comprehend 

 those stages in which takes place the formation of 

 the most important organ-systems, of the mesoblastic 

 somites, of the nervous system, and of the notochord 

 (Figs. 35-53). 



From the standpoint of philogenetic consideration 

 this period may be divided into two sections. Of 

 these the one comprises the formation of the meso- 

 blastic somites and of the neural canal. It reaches, 

 that is to say, up to the stage of Fig. 47. The second 

 period, on the other hand, in which these organ- 

 systems attain to still sharper separation and further 

 formation, is characterized principally by the formation 

 of the notochord. 



The processes with which this chapter deals were in 

 their important points well described by Kowalevsky 

 in his "Further Studies." My own observations con- 

 stitute therefore for the most part only a further 

 prosecution of his ideas. The most important differ- 



