210 Dr. J. Barrois on the, 



The inner lamella thus produced is destined to form in the 

 thorax the two pairs of cseca of the stomach, and in the abdo- 

 men the whole of the hepatic mass. The wall of the cseca of 

 the stomach is formed at once by the simple fact of the 

 appearance of a layer of cells at the surface of the two masses 

 of nutritive vitellus {cce, fig. 6) ; the vitellus which fills them 

 is gradually absorbed. As regards the liver, the cells of the 

 inner lamella begin by penetrating into the vitelline mass, at 

 first skirting the diaphragmatic partitions and then the 

 internal organs formed at the expense of the ventral meso- 

 dermic mass of the abdomen. In consequence of this pene- 

 tration of the cells into the interior, the nutritive vitellus 

 becomes divided into isolated fragments ; and these are gradu- 

 ally absorbed in proportion as the surrounding endodermic 

 cells multiply and take their place. 



The liver and the two pairs of cseca of the stomach are 

 therefore all developed at the expense of the inner lamella, 

 while the straight tubular portion of the digestive canal, com- 

 prising the oesophagus, the sucking-stomach, the intestine, 

 and the rectum, is developed at the expense of the two other 

 lamella?. The first two alone represent the endoderm ; and it is 

 evident that they only form a single part divided into two by 

 the constriction of the thorax ; in the Arachnida with an 

 unpedunculated abdomen (Chelifer, Scorpions) they are not 

 separated from each other. 



Beyond this formation of the inner lamella I have nothing 

 to describe, except some phenomena of detail which it would 

 be tedious to enumerate here (differentiation of the ventral 

 mesodermic mass of the abdomen, formation of the eyes, esta- 

 blishment of the circulation, &c). The most important 

 phenomena consist in the strong development of the peripheral 

 musculature (m' y fig. 6), especially in front of the cerebral 

 ganglia. At the commencement the internal organs, nervous 

 masses, stomach, &c, are closely applied to the skin, from 

 which they are separated only by a thin layer of mesodermic 

 cells ; the cheliceree are then still immediately anterior to the 

 cerebral ganglia; and the anterior triangular plate of the 

 thorax, which may still be distinguished even in the adult, 

 evidently corresponds to the cerebral ganglia. Subsequently, 

 when the thin mesodermic layer which clothes these organs 

 has become developed into a powerful musculature, the inter- 

 nal organs are displaced inwards, and become greatly sepa- 

 rated from the skin, which renders the relations of the divi- 

 sions of the body to the internal organs more difficult of 

 recognition. The chelicerse depart far from the ganglionic 

 masses ; and an entire long anterior portion seems to be added 



