ZOOLOGY AND BOTANY, MICEOSOOPY, ETC. 181 



muscles, the pericardium, and the sexual organs. In the bud of 

 Perophora all these parts arise from the blood-corpuscles contained 

 between the epi- and hypoblast. 



In the adult of Perophora the wall of the heart is unilaminar ; 

 and the protoplasm of the deeper parts of its constituent cells takes 

 on the structure of muscular fibrils. There is no cardiac endothelium. 

 The wall of the heart is only a continuation of the visceral fold of 

 the pericardium. This comes about from the fact that the mass of 

 mesodermic cells from which the pericardium is developed is bilaminar, 

 and a cavity appears between the laminae, forming the pericardiac 

 cavity ; the inner lamina encloses a chamber which fills with cor- 

 puscles, and it becomes the wall of the heart. 



In the primitive mass of mesodermic cells destined to form the 

 sexual organs an excentric cavity appears, becoming the sexual vesicle ; 

 this divides into an exterior, female, and an interior, male portion ; 

 both are hollow and open into a long common tube formed of flat cells, 

 which lies between the intestine and the gastro-oesophageal part 

 of the alimentary canal, ending blindly at each extremity. This tube 

 by growth becomes folded on itself, and its external section becomes 

 the oviduct, its inner one the vas deferens ; the posterior inflated end 

 of the latter becomes the testis, which, single at first, becomes multi- 

 lobate. The ovary arises by the conversion into germinal epithelium 

 of the flattened epithelium of the posterior end of the oviduct ; the 

 primitive ova thus formed become imbedded in the investing con- 

 nective tissue and form a follicular mass. The ovum falls into the 

 oviduct when mature. At first the vas deferens opens into the oviduct, 

 but when the csecal anterior end of the latter opens into the cloaca, 

 the opening of the former reaches the cloaca also and becomes inde- 

 pendent. The strong analogies which exist between the development 

 of the pericardium and the sexual vesicle show that if the pericardiac 

 chamber is homologous with that of Vertebrata, that of the sexual 

 organs corresponds with the abdominal cavity. 



The body-cavity (" enterocele ") of the larva completely disappears, 

 for the epithelial cells which line it expand into a " blastocele," and 

 then form a continuous mass, or mesenchyme. There is thus no 

 radical distinction between the mesoderm and mesenchyme as held by 

 the brothers Hertwig to be the case. In their structural characters 

 and in the mode in which the nerves terminate in them, the muscles 

 of the adult approach the smooth muscular tissue of Vertebrata, but 

 those of the heart are peculiar in consisting of parallel fibrils placed 

 in the deeper parts of epithelial cells. 



It follows from the above facts that the mesenchyme has not 

 always the same origin or the same anatomical importance in the 

 animal kingdom. In the Coelenterata and Vertebrata it is a primitive 

 mesenchyme, as being produced by contact with an epithelium ; in the 

 Ascidians it is secondary, for it results from the dissociation of the 

 cellular elements of an epithelium (the original mesoderm). The 

 muscular fibres which originate from cells of the mesenchyme 

 appear to be always fibre-cells, whether the mesenchyme is primitive 

 or secondary. 



