144 BIOLOGICAL LECTURES. 



I have already described to you the teloblastic growth of the 

 metanaupliar region of the Isopod embryo, in so far as the 

 ectoderm is concerned in it, but careful observation will show 

 that the mesoderm also participates in this mode of growth. 

 Lying below the ectodermal teloblasts or ectoblasts, as they 

 may be termed, one may see, as is shown in Fig. n, a row of 



et *" " 



eight mesoblasts (mt\ and in embryos of the proper age in front 

 of these may be seen a number of rows (m), also consisting of 

 eight cells, which have been budded off from the mesoblasts. 

 The mesoblastic rows are separated from one another by inter- 

 vals corresponding to a single row of ectodermal cells, and it 

 must therefore be concluded that the ectoblasts divide twice as 

 frequently as the mesoblasts. By tracing on the development 

 it will be found that each mesoblastic row corresponds to a 

 metamere of the adult animal, the mesoblasts themselves form- 

 ing the mesoderm of the terminal metamere or telson. Now 

 there are in the metanaupliar region of an Isopod two maxillary, 

 one maxillipedal, seven thoracic, and six abdominal metameres, 

 in all, together with the telson, seventeen metameres. Since 

 the mesoderm of each of these metameres is developed from 

 the products of a single division of the mesoblasts, and since 

 the mesoblasts themselves develop into the mesoderm of the 

 telson, it is evident that the mesoblasts must divide in a telo- 

 blastic manner just sixteen times, and no more and no less ; 

 otherwise there would be an extra metamere, or else a missing 

 one in the adult animal. 



The constancy of the number of metameres in the Isopods 

 and their allies is remarkable, and I know of no recorded cases 

 of variation in this respect in the group. One teloblastic 



