404 EDWIN G. CONKLIN 



EXPLANATION OF FIGURES 



The drawings were all made from sections of eggs and embryos of Phallusia 

 mamillata, with the single exception of fig. 9, which is a drawing of a whole mount; 

 all drawings were made with the aid of the camera lucida and as reproduced 

 represent a magnification of about 300 diameters. The mesodermal crescent and 

 the parts to which it gives rise are shaded by lines; in figs. 1 and 2 the yolk is repre- 

 sented by spherules and the protoplasm by stipples, in the remaining figures the 

 stippled areas represent the ectoderm, the cells of the neural plate being more 

 closely stippled than the general ectoderm; the yolk in the endoderm cells is 

 merely indicated by a crenated line around the nuclei, which line is lacking in 

 the chorda cells. 



1 Antero-posterior section through one of the blastomeres of the 2-cell stage, 

 taken parallel to the first cleavage plane, showing the mesodermal crescent the 

 central protoplasmic area surrounding the nucleus, and the peripheral yolk; the 

 latter is chiefly aggregated in an area anterior to the mesodermal crescent, which 

 ultimately gives rise to the endoderm; on the anterior margin of the egg, below 

 the equator, is a deeply staining crescentic area which gives rise to the neural 

 plate and chorda. 



2 Antero-posterior section, at right angles to the first cleavage plane of an 

 egg during the second cleavage. The mesodermal crescent, yolk and central 

 protoplasm are shown as in the preceding figure. 



3 Oblique antero-posterior section through an elongated ^ gastrula. The 

 ectoderm has not overgrown the injured side. Only a portion of the muscle cells 

 of one side are shown, those of the other side are entirely lacking. 



4 Horizontal longitudinal section of ^ larva showing the manner in which 

 the ectoderm sometimes closes by the infolding of the injured side. The notochord 

 is typical in form but only half the normal size, and the muscle cells are found on 

 one side only of the notochord. 



5 Horizontal longitudinal section of ^ larva, taken near the dorsal side, show- 

 ing gastric cavity completely surrounded by endoderm, a row of muscle cells of 

 the left side, the dorsal ends of a few chorda cells (unshaded), and a solid mass of 

 neural plate cells, containing a large spot of eye pigment. 



6 Oblique cross section taken near the base of the tail of a ^ larva. Neural 

 plate cells (stippled) lie at the upper side of the figure, a caudal endoderm cell 

 near the lower side and between the two is a double row of chorda cells (unshaded) ; 

 muscle cells are found on the right side only. 



6b Cross section of the tail of a § larva, with a large chorda cell in the center, 

 and with three muscle cells on its right, and two neural plate cells and a caudal 

 endoderm cell on its left; the neural plate cells, marking the dorsal mid-line, and 

 the caudal endoderm cell, marking the ventral mid-line, have come into contact 

 on the injured side, and there is no trace of muscle cells between them. 



