BIGELOW: EARLY DEVELOPMENT OF LEPAS. 133 



lack of a definiteness of statement sufficient to afford basis for com- 

 parisons of any value. 



Comparing the development of Astacus with that of Lepas, the ecto- 

 blastic mesoblast at the anterior edge of the blastopore appears to be 

 equivalent to the " secondary mesoblast " of Lepas. If the suggestion, 

 that the invagination is mes-entoblastic, proves true, it may be possible 

 to regard the mes-entoblastic cell d 4 ' 1 of Lepas as representing theinvagi- 

 nated cells of the higher Crustacea ; the primary mesoblast and ento- 

 blast of Lepas would then be comparable with the germ-layers derived 

 from the invaginatiou in the higher forms. In such a case there would 

 be further agreement with Lepas in that the mesoblast originates from 

 both ectoblast and entoblast. 



Summary. 



1. Lepas resembles most other Crustacea (a) in respect to position of 

 the blastopore, which is ventral and posterior, (b) in extension of the 

 entoblast and mesoblast from the blastopore as a starting-point, (c) in 

 the mode of formation of the organs of the larva. 



2. In Lepas, as in most other Crustacea, the mesoblast and entoblast 

 originate in the region of the blastopore from cells which, speaking in 

 general terms, at first lie in the blastoderm and later migrate into the 

 cleavage cavity. 



3. Among the migrating mes-entoblastic cells one can distinguish in 

 Lepas the individual cells of eutoblast and of two varieties of meso- 

 blast. Eepresentatives, if not precise homologues, of these kinds of 

 cells are probably present both in other Entomostraca and in the higher 

 Crustacea. 



XII. General Summary with Table of Cell-Lineage of Lepas. 



The results which are of special interest in relation to the develop- 

 ment of Cirripedia have already been summarized in connection with 

 the accounts of the several stages of development. Only results of 

 more general interest are again summarized here. 



The cleavage of Lepas is throughout total and unequal. 



Stages with 2, 4, 8, 16, 32, and 62 "resting" cells are regularly 

 formed. 



In the eight-cell stage and thereafter there is a well-marked bilateral 

 arrangement of the cells. 



In the first three cleavages three " protoplasmic " micromeres are 



