248 The American Naturalist. [March, 



the mesenteron is really formed by cells of the entodermic plates. This 

 interpretation would be more in line with what is known of other Crus- 

 tacea. Some of the most interesting observations and conclusions of 

 the paper are those concerning the development of the metanaupliar 

 regions of the embryo. It is a remarkable fact that the Naupliar and 

 - regions are very sharply distinguished by 

 . Dr. Patten was the first to call attention 

 to the fact of teloblastic growth in the ectoderm and mesoderm of 

 Cymothoa. Dr. McMurrich has gone further, and in his comparative 

 study, has made out in detail the character and limits of this method 

 of growth in Isopods. While the Naupliar is formed as described, the 

 metanaupliar regions, are the result of teloblastic growth in ectoderm 

 and mesoderm, just as the metatrochophoral regions of Polygordius are 

 due to a similar process. The author is inclined to regard these two 

 instances of teloblastic growth as acquired independently. He thinks 

 that in the Isopod " the development points back to a period where a 

 free-swimming Nauplius occurred in the development of the ancestors 

 of the group, the egg embryo being a nauplius." At such a time the 

 metanaupliar regions were developed after hatching. Now, however, 

 this posterior region is developed in Isopods before hatching, but it still 

 retains the peculiar teloblastic method of development, and is sharply 

 distinguishable from the Naupliar area. 



There is unfortunately not space to describe this remarkable process. 

 It is interesting to note, however, that, while the ectoderm of the 

 metanaupliar regions arises from the successive divisions of a row of 

 ectodermal teloblasts, the rythm of these divisions is not the same as 

 that of the row of mesodermal teloblasts which lies beneath. The meso- 

 dermal teloblasts divide just 16 times giving rise to 16 transverse rows 

 of mesoderm cells, "each of which rows is equivalent to a segment," as 

 is proved on the appearance of appendages. The ectodermal teloblasts 

 divide twice as many times. 



Though these are the main points of the paper, a number of impor- 

 tant observations and conclusions have been necessarily crowded out 

 of this review. For instance, I have not touched on the processes of 

 impregnation, the formation of membranes, the details of segmentation 

 and differentiation, the formation of the digestive tract, the history of 

 the vitellophages, or the development of certain organs. 



-H. McE. Knower. 



