362 W. F. R. WELDON. 
way to suggest a possible reason for the extremely different 
results at which we have arrived. . 
The account here given of the early development suggests 
many interesting comparisons with corresponding stages of 
other forms, and especially with Lopadorhynchus, but I 
prefer to postpone a discussion of these points until I am in a 
position to describe the later history of Crangon. 
EXPLANATION OF PLATES XX, XXI, and XXII, 
Illustrating Professor Weldon’s paper on “ The Formation of 
the Germ-layers in Crangon vulgaris.” 
List of Reference Letters. 
Ant.i. First antenna. Ax. ii. Second antenna. Blp. Blastopore. c.0.p. 
circumoral portion of ventral plate. 2b. Ectoblast. Hz’. Endoderm which 
is just invaginated as scattered cells. Hz'. Continuous endoderm of later 
invagination. 1/6. Mesoblast. Md. Mandible. me. Mesoderm. x. m. p. 
Neuro-muscular plate. NW. 8. Nerve-cord. o. g. Optic ganglion. 0. p. 
optic plate. r.p. retinal plate. Stom. Stomodeum. Th. abd. Forecast of 
the thoracico-abdominal segments. 
Fies, 1—4.—External views of segmentation. 
Fie. 5.—Embryo with newly-formed blastopore. 
Fic. 5a.—Outline of embryo slightly older than Fig. 5, showing the 
crowding of nuclei in the region of the future ventral bands. 
Fie. 6.—Embryo with fully-formed ventral bands and conspicuous blasto- 
pore. 
Fic. 7.—Embryo with optic plate, first antenne, and mandibles. 
Fig. 8.—Embryo with Nauplius appendages, before the formation of an 
abdominal flexure. 
Fie. 9.—Fully-formed Nauplius, after ecdysis, with well-marked abdo- 
minal flexure. 
Fie. 10.—Section through an eight-celled egg. 
Fic. 11.—Section through an embryo with sixteen cells. 
Fic. 12.—Section through an embryo with 128 cells. 
