84 BULLETIN OF THE 



the folded endoderm have not yet met, so that the ventral side of the 

 gut still presents an opening at this place. 



It is difficult to distinguish the different parts of the mesoderm (proto- 

 vertebrse and lateral plates) in the region anterior to the ear capsule ; 

 posterior to this region, however, they are plainly definable. 



At four days (Fig. 3) the closure of the gut has been effected farther 

 forward, and from this time on the conditions at the level of the heart 

 region are complicated by the formation of gill slits. The gut, which 

 elsewhere is a tube only slightly flattened dorso-ventrally and exhibiting 

 a considerable lumen, as shown in Figures 3 to 5, now becomes greatly 

 flattened dorso-ventrally in the gill region, where its walls consist of thin 

 cells so closely applied to one another as to obliterate its lumen ; it here 

 sends out lateral evaginations on either side, which meet corresponding 

 invaginations of the ectoderm, the union with which establishes a direct 

 communication of the gut with the outside. Diff'erent stages in the for- 

 mation of the gill slit may be seen in Plate II. Figs. 6-8. As the lateral 

 plates now begin to move toward the median plane, the gut in this region 

 is forced up against the chorda and toward the neural tube, and is always 

 found to consist of two layers of cells pressed against each other so as to 

 leave scarcely any lumen discernible between them. At the same time, 

 the approaching median edges of the lateral plates, which at first con- 

 sisted of onh" a single layer of cells, become thickened until they are two 

 or three layers deep. However, it is only at the median fold of the lat- 

 eral plate, where the two layei*s constituting the inner and outer walls 

 of the lateral mesoderm bend over into each other, that the thickening 

 occurs. The thickness of the lateral plate gradually diminishes laterally 

 luitil it is composed of only two cell layers separated more or less dis- 

 tinctly by the primitive body cavity (Plate II. Figs. G-8). 



In many instances the lateral plates in the heart region cannot be traced 

 as far from the median plane as the outer margin of the embryo (margin 

 of neural tube and protovertebra'), and, if distinguishable at all, are repre- 

 sented by only a single layer of cells instead of two (Plate II. Figs. 6 and 

 7). Farther posterior they extend out beyond the lateral limits of the 

 protovertebrse (compare with Figs. 4 and 5). Embryos from five to six 

 days old (Figs. 6 and 7) show that the plates of the two sides of the 

 body are approaching each other symmetrically, and that there are be- 

 tween their approaching bent ends a considerable number of loose cells 

 (^. c. m.). Figure 6 shows that the approaching edges of the plates are 

 becoming thicker, the thickening of the left-hand plate being as yet the 

 result of the enlargement of the individual cells onlv, whereas the right- 



