THIRD PERIOD OF DEVELOPMENT. 83 



On the section from the most anterior region of 

 the body (Fig. 86) nothing is to be seen of the 



formation of mesoblastic somites. 



Let us now consider a section through the first 

 mesoblastic somite (Fig. 87). "We see that here the 

 folding is still much sharper defined than before. 

 The lumen of the mesoblastic somite is in continuous 

 connexion with the cavity of the archenteron. 

 The series of cells of the mesoblastic somite is seen 

 in its arrangement to be already quite out of con- 

 tinuity with the hypoblast plate. The cells of the 

 latter are at the slit where the mesoblastic somite 

 lumen, opens inwards already about to unite with 

 one another, whereby the mesoblastic somite becomes 

 completely separated from the hypoblast. The 

 form too of the cells of the mesoblastic somite has 

 altered, while these cells, as opposed to the columnar 

 ones of the archenteron, have assumed a low and 

 more cubical form. A histological differentiation too 

 makes itself in so far noticeable that in the cells of 

 the mesoblastic somite the yolk granules are dissolved 

 more quickly than in the hypoblast. This becomes 

 noticeable in comparison with the hypoblast cells 

 through their not being made so brown by osmic 



