MORSE: EARLY STAGE OF ACMAEA. 317 
A line drawn through the cicatrix of alveiis longitudinally varies a 
few degrees from a similar line drawn through the permanent shell, 
the direction indicating that the embryo shell was turned to the left 
as in testudinalis. 
The primitive condition of the embryo shell, as we have seen it in 
the two species of Acmaea studied, with no trace of a coiled nautiloid 
shell, at least, in testudinalis, is certainly a low character and is corre- 
lated with many low characters of the order. By all students the 
Patelloidea and its allies have been regarded as among the most 
primitive gastropods. In running through the raolluscan part of 
Korschelt and Heider's "Text-book of the embryology of inverte- 
brates," (5), revised by Professor Woodward, nearly every reference to 
Patella emphasizes the archaic character of the group. I must here 
acknowledge my indebtedness to this compendium of embryological 
research. I can do no better than to transcribe briefly the few refer- 
ences to Patella that the student may appreciate the relations between 
this genus at least and Mollusca far below Patella in classification. 
In speaking of the row of ciliated cells and the frontal or apical tuft 
in the embryo Chiton, Korschelt says, "Very similar embryonic stages 
are met with in the ontogeny of other mollusca, i. e. Patella." Patten, 
in his memoir on Patella (9) calls attention to the exceedingly simple 
method of its oviposition. The eggs are laid singly and are apparently 
fertilized in the water as copulatory organs are wanting in this genus. 
Korschelt remarks that the early larval stages of Dentalium closely 
resemble those of Patella and also points out that in Gastropoda 
"gastrulation is attained in different ways in accordance with the 
variation in cleavage. In the simplest cases, e. g. Planorbis and 
Patella a blastula with a comparatively large cleavage cavity arises. 
The vegetative pole of the blastula is formed by the macromeres and 
consequently appears much thickened. After the mesoderm has 
become differentiated, the entomeres begin to increase in number, 
and the whole entoderm becomes invaginated into the cleavage- 
cavity, and thus a typical invaginated-gastrula forms. In Patella, 
on the contrary, an extremely large solid ingrowth of macromeres 
takes place from the vegetative pole of the blastula. From this in- 
growth, the mesoderm and entoderm become differentiated and, at a 
later period, an archenteric cavity forms within the till now solid 
entoderm." 
"The ontogeny of Patella shows primitive conditions in so far as 
