BIOLOGICAL LECTURES. 



equally, in all cases of teloblastic growth, for instance, the 

 division of the teloblasts being unequal ; and, indeed, the phe- 

 nomena presented by teloblastic growth stand apparently in 

 such marked contrast to what might be expected from the opera- 

 tion of the factors included in Sachs' law that a brief consider- 

 ation of them will not be out of place here, and for exam- 

 ple's sake the case of teloblastic growth seen in the embryo of 

 an Isopod Crustacean may be considered. 



In an Isopod embryo, such as that of Asellus, two well- 

 marked regions can be distinguished. Anteriorly there is rec- 

 ognizable a somewhat heart-shaped region whose ectoderm 

 shows more or less distinctly the orthogonal trajectories already 

 referred to, and which, as the later development shows, is that 

 portion of the embryo which corresponds to the Nauplius larva, 

 which is of such frequent occurrence in the life-histories of the 

 lower Crustacea, but which does not exist as a free-swimming 

 stage in the Isopods, there being in these forms a marked con- 

 densation of the development. Behind this naupliar region 

 one finds the ectodermal cells arranged in remarkably definite 

 longitudinal rows, varying in number from about twenty-two to 

 twenty-five, and on tracing them back towards the hind end of 

 the embryo each one will be found to terminate in a single 

 large cell which is known as a teloblast (Fig. 1 1, et). It is by 

 the continued division of these teloblasts that the cell-rows are 

 formed ; in a very young embryo the teloblasts are situated at 

 the posterior end of the naupliar region, the metanaupliar 

 region, as it is termed, being unrepresented at this stage, and 

 if successive stages be examined it will be found that first of 

 all spindles form in the teloblasts with their long axes parallel 

 with the longitudinal axis of the embryo, and situated slightly 

 in front of the centers of their cells, the result being that a 

 transverse row of cells is divided off from the teloblasts by a 

 process of unequal division. Later spindles again form in the 

 teloblasts, and another transverse row of cells is interposed 

 between the teloblasts and the row previously formed, and so 

 the process goes on, the teloblasts being gradually forced back- 

 wards over the surface of the yolk as the transverse rows 

 increase in number. 



