THIRD PERIOD OF DEVELOPMENT. 113 



We can, through the study of whole embryos, 

 at once recognise that the cavity of the anterior 

 mesoblastic somites is completely shut off from that 

 of the archenteron (Fig. 49). Also 011 a consider- 

 ation of the embryos from the dorsal surface, it may 

 be recognised that the cavities of the mesoblastic 

 somites are enlarged through flattening of the cells 

 (Fig. 49, 52). 



At once the mesoblastic somites penetrating between 

 epiblast and hypoblast broaden out ventral-wards. 

 This may be followed step by step in the representa- 

 tions of an embryo with seven mesoblastic somites, 

 Fig. 48, and also one with nine, Fig. 50. 



Further, in the arrangement of the cells a condition 

 has made its appearance which arouses our special 

 attention. The cells, which are situated towards the 

 epiblast, and those which form the division walls 

 between, the individual mesoblastic somites (Fig. 52) 

 appear mostly flattened. Those seem higher which 

 are in contact with the notochord. These last are 

 arranged now, that is, while the complete abstric- 

 tion of the mesoblastic somites is taking place, in such 

 a manner that they are continuous through the whole 

 length of the mesoblastic somite. This is to be ob- 



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