I02 MCMC/RRICH. [Vol. XI. 



tained. Here the region occupied by the mes-endoderm has 

 greatly increased, being represented in the figure by the shaded 

 area, though it is probable that a portion of the area really lies 

 outside the true mesendodermal region, the anterior margin of 

 the area appearing dark in the preparation on account of the 

 lower layer mesendodermal cells having begun to migrate for- 

 wards and to scatter beneath the general surface of the blasto- 

 derm. This forward migration and scattering has proceeded 

 further in Fig. 57, which is from Arniadillidiuni, though prepa- 

 rations similiar to this and the succeeding figure were obtained 

 from Porcellio. The concentration of the cells to form the 

 blastoderm has now reached its greatest extent, the nuclei 

 are crowded together, and the entire area covered by the blas- 

 toderm, notwithstanding the increased number of cells, is 

 little if any greater than that which it covers in the stage rep- 

 resented in Fig. 53. A little behind the middle of the 

 blastoderm is seen a very dark area {MEn) which represents the 

 mesendodermal region in which the lower layer cells are heaped 

 together to form a mesendodermal plug projecting into the 

 yolk, and anteriorly, as well as posteriorly, less dark areas are 

 to be seen, which are due to lower layer or mes-endoderm cells 

 which have migrated forwards and backwards from the mes-en- 

 doderm region. I say migrated, because I have not been able 

 to find any evidence for the assumption that they originate in 

 situ by delamination from the ectodermal portion of the blas- 

 toderm. Numerous spindles are to be found in blastoderms of 

 this stage, but I have never seen any directed perpendicularly 

 to the surface, and believe that here as in Jaera and Aselhis all 

 the mes-endoderm comes from the cells of the mesendodermal 

 region. 



In this figure there is to be seen just at the front edge of the 

 mesendodermal region an imperfectly defined row of cells which 

 are the first indication of the ectodermal teloblasts (7^). They 

 are not sufficiently well marked to determine their exact num- 

 ber, though they seem to be more numerous than eleven, which 

 was the number first differentiated in Aselhis. They serve to 

 mark accurately the anterior edge of the mesendodermal 

 region. 



