PLATE XII. 



Development of Decapoda, continued. Figures from T. H. Huxley, N. Bobretzky, Heixricii 

 Reichenbach, Heinrich Rathke, Walter Faxon, William Stimpson, Alexander 

 Agassiz, and Paul Mayer. 



a. Anus. P '• Procephalic lobe. 



n b. Abdomen. >■ <"• External branch of appendage. 



br. Gill. ri. Internal " " 



cp. Carapace. ^ d. Stomodieum, or fore-gut. 



ep. Epiblast. sp- Spine. 



dp. Deutoplasm. "• Yolk. 



V "^■ 



Gastrula mouth. v m. Vitelline membrane. 



h. Heart. "• Epithelium of ovisac. 



h y. Hypoblast. P- Membrana propria. 



lb. Labrum. Y- Stalk of ovisac. 



m. Mouth. *• Basal portion of abdominal appendage. 



m e. Mesenteron, or mid-gut «. Inner branch " " 



ms. Mesoblast. ?■ Outer " " " 



n. Ventral nerve cord. i- Egg-case. 



n'. Supra-cesophageal nerve ganglion. ,>. Median spine. 



n c. Nucleus. '• Lateral spine. 



oc. Eve. Z'. Mesoblast cell splitting off from hypoblast cell. 



p d. Proctodaemn, or hind-gut. 



The Roman numerals denote the appendages in their consecutive ordei*. 



1 - 10. Development of Astacns. 



1-3. From Huxley, The Craytisli, London and New York, 1880. 



1. Spermatozoon of Askicus fluviatilis developing in a seminal cell, x 850. 



2. Mature spermatozoon of the same, viewed en face. 



3. Two-thirds grown egg of the same, contained in its ovisac, o, epithelium of ovisnc. /3, memhrana propria, 



or structureless membrane investing the ovisac. vvi, vitelline membrane. v, yolk, nc, germinative 

 vesicle containing germinative spots. 7, stalk of ovisac. 

 4-7. From Bobretzky," Kl) BMBPIO.lOnil q.TEffllCTOHOrnX'L. San. Kies. 06. Ect., III., T. I., 1873. 

 [On the Embryology of Arthropods, Mem. Kiel! Natuialists' Soc, III., PI. I., 1S73.] 



4. Portion of egg of Astacns in the gastrula stage. d }}, food-yolk, cjm, gastrula month, c p, epiblast. 



hy, hypoblast, ms, mesoblast. 



5. Smaller portion of the same, more highly magnified, to show the origin of the mesoblast cells, m, mesoblast 



cell splitting off from one of the hypoblast cells at the mouth of the gastrula cavity. 



Note. — According to Keichenbach 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 



(»;.<;). 0.6, abdomen, p 0?, proctodeum, or hind-gut. s d, stomoda-'uni, 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 jjyramids 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 (oesophagus and .stomach) and hind-gut (posteiior part 

 of the intestine) have not yet opened into the mid-gut. h, heart, formed in the niesoblastic tissue. 11', n, 

 supra- and sub-a-sophageal portions of nervous system, fonned from the epiblastic germ-layer, c p, fold 

 which forms the carapace. 



8. Nauplius stage of the embryo of Astacus fluviatilis. I, first antenna. 11, second antenna. Ill, mandible. 



Z 6, labrum. a, anus, cy, carapace. ;>?, procephalic lobes. c, optic pit, epibla.stic invagination in the 

 procephalic lobes concerned in the formation of the supra-oesophageal ganglion and nervous elements of 

 the eye. li, heart. From Pieichenbach, Die Embryonalanlage und erste Entwicklung des Flusskrebses. 

 Zeitschr. wissensch. Zool., XXIX., Taf. X. fig. 8, 1877. 



