MUSEUM OF COMPARATIVE ZOOLOGY. rom 
The spinning glands are not yet definitely established, but in the anal 
region on the ventral side of the proctodzum there is a large accumula- 
tion of ectodermic cells (Pl. XI. fig. 70) from which they are subse- 
quently developed. 
Late in this period the infoldings for the lungs arise. There appear a 
pair of large oval masses of cells, the nuclei of which are arranged in par- 
allel lines (Pl. XI. fig. 73). From these cells the respiratory lamelle of 
the lungs are finally formed. 
The mesoderm likewise has been growing rapidly during reversion. 
In the previous period it was confined to the ventral portion of the em- 
bryo, but during the present period it grows upward on either side until 
it reaches the dorsal median line, thus forming a continuous layer be- 
neath the ectoderm, as well as an investment for all organs, which arise 
as outgrowths of either ectoderm or entoderm. 
The dorsal growth of the rudimentary terga, already spoken of as 
external features, is followed by this underlying layer of mesoderm. 
Early in the formation of the dorsal elements this mesodermic layer is 
divided into corresponding somites. Balfour (80, p. 181) concluded 
that the cells out of which are formed the dorsal somites of the meso- 
derm ‘‘are not derived from prolongations of the somatic and splanchnic 
layers of the already formed [ventral] somites, but are new formations 
derived from the yolk.” My sections, however, indicate that there is a 
direct continuity between the two (Pl. IX. figs. 59, 61), and that the 
dorsal mesoderm is an outgrowth from the previously established ventral 
mesodermic somites. 
It is during this period also that the heart is formed. While I have 
been unable to arrive at an entirely satisfactory understanding of the 
details of its formation, I am convinced that it is not, as Balfour states, 
developed from a solid cord of cells, but from the dorsal limb of the up- 
growing mesoderm, and that its dorsal wall is closed first, while the 
ventral wall—the floor —remains for a time widely open below, thus 
communicating freely with the yolk. My sections also show that ata 
later period the aorta is formed, by means of a constriction, from the 
mesenteron. This agrees with the recent observations of Schimkewitsch, 
(84°). 
A layer of characteristic cells, to which Balfour alludes in speaking of 
the formation of the dorsal mesoblast, precedes the formation of the 
heart in the dorsal region. These are what have been called “ primary 
entoderm ” cells, and are sharply distinguished from the surrounding cells 
by their large size, their large, oval nuclei, and their yellowish tint. These 
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