Io INTRODUCTION. 
ceeding gradually backward. Usually the closure is complete, but 
occasionally the hinder part remains open and forms the anus. Where 
it closes completely the vent is subsequently formed in the line of closure. 
This union of the two lips of the blastopore in closing marks the middle 
line of the back of the future animal, and is called at first the primitive 
groove, the region on either side of it being known as the primitive 
streak, terms of importance in understanding the gastrulation of the 
higher vertebrates. 
Mesoderm.—With the closure of the blastopore the embryo elon- 
gates and the archenteron is converted into a tube. Next, from the 
region of closure and from the entodermal tissue, a fold of cells grows 
in on either side between ectoderm and entoderm, thus farther en- 
croaching on the segmentation cavity. These cells form the middle 
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Fic. 5. Fic. 6. 
Fic. 5.—Stereogram of the anterior end of a developing amphibian, showing the out 
lining of the mesothelium, nervous system and notochord. 4, anterior end; a7, archenteron; 
c, cceelom; ch, notochordal cells; ec, ectoderm; mp, mesodermal pouch; ng, primitive groove; 
np, neural plate; mr, neural folds; sc, segmentation cavity; so, somatic wall of celom; sp, 
splanchnic wall of ccelom. 
Fic. 6.—Stereogram of the anterior end of a vertebrate, showing the relation of the 
coelomic pouches; ¢, ccelom; d, digestive tract; e, ectoderm; nc, nervous system; m, notochord; 
sc, segmentation cavity; so, somatic and sp, splanchnic walls. 
layer ormesoderm. Inside this fold is a space, connected at first with 
the archenteron, but soon the cavity of each side is cut off by a growing 
together of the opening into the archenteron and is henceforth known 
as a coelom’ or body cavity. Each coelomic space has two walls, one 
toward the ectoderm, the somatic layer, the one toward the entoderm 
being the splanchnic layer (figs. 5 and 6). 
The mesoderm arising in this way and bounding the ccelom is 
called mesothelium to distinguish it from another kind—the mesen- 
1 4 ceelom formed in this way is an enteroccele. Usually the coelomic walls arise as a 
solid mass of cells from the corresponding region, which later splits internally, forming a 
schizoceele. The two are readily compared. 
