BIGELOW: EARLY DEVELOPMENT OF LEPAS. 123 



was evidently based upon Nassonow's figures of Balanus ; but is shown 

 to be erroneous by subsequent investigations. It is controverted in the 

 case of Balanus, by the account of Groom, as well as by unpublished 

 observations of my own ; and in the case of Lepas it is clearly inappli- 

 cable. In both these genera cleavage is total and unequal. 



Knipowitsch ('92) described the cleavage of the Ascothoracidan genus 

 Laura as superficial from the very beginning of development. His figures 

 do not warrant such a conclusion, for cell -boundaries appear to form after 

 every nuclear division. The few figures of segmentating eggs in Knipo- 

 witsch's paper resemble the figures which other authors have drawn from 

 the eggs of parasitic copepods ; for example, Pedaschenko's ('93) figures 

 of Lernsea. The latter is evidently a case of total, but very unequal, 

 cleavage, and the cleavage of Laura is apparently to be interpreted in 

 the same way. 



Van Beneden's ('70) figures illusti-ating his account of the develop- 

 ment of Sacculina indicate to my mind that the cleavage of Ehizoce- 

 phalan Cirripedia is also of the unequal total type. Even the fact that 

 in late stages the four yolk-macromeres appear to fuse does not support 

 the interpretation that the cleavage is in later stages superficial. In no 

 stage of the development is there nuclear division which is not associated 

 with total cell division, and we are led to the conclusion that the cleavage 

 of Sacculina cannot be correctly characterized as superficial in any stage. 



Eegarding the type of cleavage of cirripede ova, the conclusion is that, 

 so far as present knowledge extends, the eggs undergo unequal total 

 cleavage, and with respect to the cleavage processes there is no close 

 resemblance to the superficial cleavage of the higher' Crustacea ; rather 

 is the resemblance to that of the yolk-laden eggs of gasteropods. 



In the order of the cleavages involved in the establishment of the 

 germ-layers there are in Lepas some interesting resemblances to the 

 annelids and mollusks. As is well known, studies of the cell-lineage of 

 annelids, gasteropods, lamellibranchs, and chitons have shown that in 

 all of these forms the ectoblast is separated from the raes-entoblast by 

 three successive cleavages, while a fourth cleavage separates the primary 

 mesoblast from the entoblast. Moreover, it has been shown in the cases 

 of some gasteropods and lamellibranch mollusks, that the mesoblast is 

 derived from both primary germ-layers; in addition to the primary 

 mesoblast (entoblastic mesoblast) there are mesoblast cells which come 

 from the ectoblast (ectohlastic mesoblast). This has been designated 

 " secondary mesoblast " or " larval mesenchyme " (Lillie, '95, p. 24 ; 

 Conklin, '97, p. 150). 



VOL. XL. — 2 5 



