128 MCMURRICH. [Vol. XI. 



My belief, then, with regard to the vitellophags, is that they 

 take no part in the formation of the digestive tract, that some 

 of them disintegrate and disappear, but that the majority take 

 part in the formation of persistent structures, such as connec- 

 tive tissue, muscle tissue, blood corpuscles, and perhaps even 

 of the heart itself. These are what are generally recognized 

 as mesodermal structures, and I do not believe that the vitello- 

 phags should be regarded as being anything but mesoderm, 



3. General Considerations on the Formatioji of the Germ-layers 



in the Crustacea. 



I wish, in the first place, to emphasize once more the distinc- 

 tion existing between the mode of formation of the naupliar 

 and meta-naupliar regions of the Isopods, a distinction already 

 pointed out by Bergh ('93) as obtaining in Mysis. The latter 

 portion, so far as its ectoderm and the greater bulk of its 

 mesoderm are concerned, is produced by a teloblastic growth, 

 while in the naupliar region no such general method of growth 

 is pronounced. If the egg-embryo proper be regarded as con- 

 sisting of the blastopore region and the portion of the embryo 

 immediately in front of this, then the meta-naupliar region may 

 be regarded as a portion of the body normally developed after 

 hatching, but in the Isopods developed, in accordance, probably, 

 with the occurrence of a brood-pouch, before the embryo begins 

 to lead a free life. In other words, the development of the 

 Isopods points back to a period when a free-swimming N'anplins 

 occurred in the development of the ancestors of the group, the 

 egg-embryo being a Naupliiis, if one may so express it, just as 

 it is in the Penens, for example, in which the post-mandi'bular 

 segments develop only after hatching. In the Annelida a telo- 

 blastic mode of growth has been described, more especially in 

 connection with the mesoderm, and if the development of such 

 a form as Polygordins be taken as a type of the larval method 

 of development in the Annelids, an interesting comparison may 

 be made. Thus in Polygordius the Trochophore may be re- 

 garded as the egg-embryo proper, the addition of new segments 

 being associated with a teloblastic growth of the mesoderm, 



