THE MESODERMAL STRUCTURES. 309 



we must content ourselves with a brief summary of the accounts given of its 

 origin. 



According to BOBKETZKY, the arterial heart is derived from two sac-like 

 organs which first appear as cavities in the massive mesoderm near the 

 rudiment of the intestine and the yolk-sac. Round these, the cells become 

 regularly arranged, and the two sacs or vesicles thus produced then unite to 

 form the heart. The formation of the auricles has already been mentioned 

 (p. 306). 



The arteries arise as canals in the mesoderm, their limits being marked by 

 the regularity of arrangement in the cells ; at a very early period blood is 

 driven through them, in consequence of the commencing pulsations of the 

 heart. After the two sacs have united to form the heart, the two pericardial 

 sacs are said to extend towards the latter, so as to enclose the heart in the 

 same way as in other Molluscs (SCHIMKEWITSCH). The branchial hearts abut 

 on that part of the body-cavity which encloses the heart, and are also covered 

 by the peritoneum. The pericardial glands (the so-called branchial appen- 

 dage) develop from the latter as growths of the epithelium ; these glands are 

 connected with the branchial hearts and are held to be excretory organs. 

 The branchial hearts are said to be differentiated from the mesoderm at the 

 broad bases of the gills, and the whole venous system, the venae cavae (chiefly 

 the anterior vena cava) being specially noticeable in the embryo, arises as 

 lacunar spaces in the mesoderm, some of these spaces changing into actual 

 veins and others into irregular blood-sinuses (BOBRETZKY). 



SCHIMKEWITSCH attributes the origin of the blood-corpuscle to the increase 

 in number and migration of cells of the yolk-epithelium in the posterior part 

 of the body, and thus assumes for them an origin similar to that of the blood- 

 cells in the Arachnida, the latter being formed from migrating yolk-cells 

 (Vol. iii., p. 88). We refrain for the present from expressing an opinion on 

 these somewhat improbable statements. 



Chromatophores, subcutaneous tissue, musculature. The layers of 

 the mesoderm lying beneath the ectoderm become transformed into 

 the so-called fibrous layer, while the deeper layers yield the con- 

 nective tissue and muscle-fibres of the cutis and also, in any case, the 

 muscles of the external organs. The chromatophores, also, are said 

 by nearly all authors to originate here, but a somewhat different view 

 of their origin has recently been propounded. 



The time of the appearance of the chromatophores in the different 

 forms varies greatly (cf. Figs. 120 and 121, p. 263). In Loligo, for 

 instance, they appear very late, but in the Cephalopod described by 

 GRENACHER at an early stage, before the circumcrescence of the 

 yolk by the blastoderm is completed and before the organs have 

 appeared (Fig. 125, p. 268). In the last case, a very early differen- 

 tiation of these mesoderm-layers seems to have taken place. 



The chromatophores are said to be derived from mesoderm-cells 

 which are distinguished from the surrounding cells by their large 



