98 LEWIS R. GARY. 



(d) Histological Changes During Development by Laceration. 



In considering the histological changes undergone by the 

 tissues during development by pedal laceration those changes 

 which take place in the basal portions of the parent individual 

 before the fragment is torn off may be first mentioned. Andres 

 (loc. cit.) mentions that in Aiptasia lacerata the rim of the basal 

 portion of the animal becomes very opaque, and that in section 

 there seems to be an unusual number of gland cells in this region. 

 The endoderm becomes very much thickened in this region so 

 that when the fragment becomes torn off there is only a small 

 cavity left in its interior (Fig. 7, PI. II.). As may be seen in this 

 section, the thickening of the endoderm is very much greater 

 on the upper side of the fragment, i. e., that which has come from 

 the original column well. 



In the period immediately following upon the separation of the 

 fragment from the parent, there is a rapid change in the character 

 of the tissues about the orifice. All of the newly forming tissue 

 becomes very light in color due to the comparative paucity of 

 the zooxanthellae in the endoderm of the active area. The 

 mesoglea becomes comparatively thin in this region, but seems 

 always to arise as a direct prolongation of that already present 

 and not as a new secretion between the ectoderm and endoderm 

 of the new parts. 



In the cellular layers the changes are more profound. In 

 the ectoderm (Fig. 19, PI. IV.), a section through a young tentacle 

 bud, the boundaries of all of the cells except the nematocysts 

 and gland cells becomes lost so that the whole layer constitutes 

 a syncytium. The cytoplasm is uniformly granular throughout. 

 The nuclei are scattered through the cytoplasm, but still indicate 

 by their positions the fact that only definite cell walls are wanting 

 in order that the usual cell structure should be apparent. At 

 the internal boundary of the entoderm there is distinguishable 

 a layer of very fine muscle fibers. When compared with the 

 size of the nuclei and nematocyst cells, these fibers are seen to be 

 several times less than the diameter of normal muscle fibers of 

 this species of actinian. Since in the fully developed Aiptasia 

 muscle fibers exist both as processes from epithelio-muscle cells 

 and as nearly the entire component of "muscle cells" in which 

 the cytoplasmic cell body is reduced to a small mass containing 



