10 



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 



FIG. 6. 



FIG. 5. Stereogram of the anterior end of a developing amphibian, showing the out 

 lining of the mesothelium, nervous system and notochord. a, anterior end ; ar, archenteron ; 

 c, coelom; ch, notochordal cells; ec, ectoderm; mp, mesodermal pouch; ng, primitive groove; 

 np, neural plate; nr, neural folds; sc, segmentation cavity; so, somatic wall of ccelom; sp, 

 splanchnic wall of coelom. 



FIG. 6. Stereogram of the anterior end of a vertebrate, showing the relation of the 

 ccelomic pouches; c, ccelom; d, digestive tract; e, ectoderm; nc, nervous system; n, notochord; 

 sc, segmentation cavity; so, somatic and sp, splanchnic walls. 



layer or mesoderm. 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 1 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 coelom is 

 called mesothelium to distinguish it from another kind the mesen- 



1 A ccelom formed in this way is an enterocoele. Usually the ccelomic walls arise as a 

 solid mass of cells from the corresponding region, which later splits internally, forming a 

 schizoccele. The two are readily compared. 



