PLATE XII. 
Development of Decapona, continued. Figures from T. H. Huxury, N. Bosrerzxy, Heryricu 
ReicHENBACH, HetnricH Ratake, Watrer Faxon, WILLIAM Stimpson, ALEXANDER 
Agassiz, and PauLt Mayer. 
a. Anus. pl.  Procephalic lobe. 
ab. Abdomen. re. External branch of appendage. 
br. Gill. ri. Internal Le es 
cp. Carapace. sd.  Stomodzeum, or fore-gut. 
ep. Epiblast. sp. Spine. 
dp. Deutoplasm. v. Yolk. 
gm. Gastrula mouth. vm. Vitelline membrane. 
h. Heart. a, Epithelium of ovisac. 
hy. Hypoblast. 8. _Membrana propria. 
76. Labrum. y- Stalk of ovisac. 
m. Mouth. é Basal portion of abdominal appendage. 
me. Mesenteron, or mid-gut. € Inner branch w us 
ms. Mesoblast. & Outer * G Sk 
n. Ventral nerve cord. 1. Egg-case. 
n’. Supra-cesophageal nerve ganglion. 3. Median spine. 
ne. Nucleus. u Lateral spine. 
oc. Eye. Me Mesoblast cell splitting off from hypoblast cell. 
pd. Proctodeum, or hind-gut. 
The Roman numerals denote the appendages in their consecutive order. 
1-10. Development of Astacus. 
1-3. From Huxley, The Craytish, London and New York, 1880. 
1. Spermatozoén of Astucus fluviatilis developing in a seminal cell. x 850. 
2. Mature spermatozoén of the same, viewed en face. 
3. Two-thirds grown egg of the same, contained in its ovisac. a, epithelium of ovisac. 8, membrana propria, 
or structureless membrane investing the ovisac. vm, vitelline membrane. v, yolk. 2c, germinative 
vesicle containing germinative spots. , stalk of ovisac. 
4-7. From Bobretzky, Kb JMBPIO.OMM GIEHWCTOHOTMND. 3an. Kies. 06. Ecr., III., T. I, 1873. 
{On the Embryology of Arthropods, Mem. Kieff Naturalists’ Soc., LIL, Pl. I, 1873.] 
4. Portion of egg of Astacus in the gastrula stage. dp, food-yolk. gm, gastrula mouth. ep, epiblast. 
hy, hypoblast. ms, mesoblast. 
5. Smaller portion of the same, more highly magnified, to show the origin of the mesoblast cells. uu, mesoblast 
cell splitting off from one of the hypoblast cells at the mouth of the gastrula cavity. 
Norte. — According to Reichenbach there are formed later, during the nauplius stage of the embryo, 
secondary mesoblast cells by a sort of endogenous formation within the hypoblast cells on the ventral side 
of the embryo. These cells wander out from the hypoblast, spread under the epiblast, and mingle with the 
primary mesoblast cells. 
6. Later stage of the same. The gastrula mouth has closed, and the gastrula cavity has become the mesenteron 
(me). ab, abdomen. pd, proctodeum, or hind-gut. sd, stomodeum, or fore-gut. 
7. Later stage of the same. The hypoblast cells have absorbed the whole yolk, and assumed the form of long 
pyramids, enclosing the cavity of the mesenteron. The bases of these pyramids are directed outward, and 
contain the nuclei and protoplasmic portion of the cells. The protoplasmic bases of the pyramids then 
separate from the deeper portions to form the epithelial lining of the mid-gut of the adult (liver and ante- 
rior portion of the intestine). The inner portion of the pyramids becomes food-yolk in the cavity of the 
mesenteron. In the stage represented, the fore-gut (cesophagus and stomach) and hind-gut (posterior part 
of the intestine) have not yet opened into the mid-gut. , heart, formed in the mesoblastic tissue. 7’, 7, 
supra- and sub-cesophageal portions of nervous system, formed from the epiblastic germ-layer. ¢ p, fold 
which forms the carapace. 
8. Nauplius stage of the embryo of Astacus fluviatilis. I, first antenna. II, second antenna. III, mandible. 
1b, labrum. a, anus. cp, carapace. pl, procephalic lobes. oc, optic pit, epiblastic invagination in the 
procephalic lobes concerned in the formation of the supra-cesophageal ganglion and nervous elements of 
the eye. h, heart. From Reichenbach, Die Embryonalanlage und erste Entwicklung des Flusskrebses. 
Zeitschr. wissensch. Zool., XXIX., Taf. X. fig. 8, 1877. 
