886 



EMBRYOLOGY 



to indicate that even the so-called heart of the Tornaria might arise from 

 the water-vascular vesicle or, what is the same thing, from the anterior 

 coelomic sac.^ The early appearance of the organ in the Tornaria is 

 favorable to the explanation (excretory) which has been given to the 

 fully developed organ. In other divisions of the animal kingdom also 

 we see the excretory system established at a very early period. 



Likewise the organ which is interpreted as the real heart first appears, 

 according to Bateson, as a fissure in the mesodermal tissue. This fissure 

 makes its appearance between the fundament of the "proboscis gland" 

 and the intestinal diverticulum, and is only gradually surrounded by a 

 firm wall. It has not been determined whether or not it is from the 

 beginning connected with the blood-vessels of the body. The blood- 

 vessels probably arise from the mesoderm in 

 the same way as the supposed central organ. 



[The observations of authors are not in 

 agreement respecting the origin of the coelomic 

 sacs, for Spengel (No. VI., Appendix to Litera- 

 ture on Enteropneusta) and Boukne (No. I., 

 Appendix) maintain that they arise from the 

 hind-gut, whereas Morgan (Nos. III., IV.) 

 refers them to the entoderm, as was formerly 

 done. Morgan, moreover, assumes a different 

 method of origin in the different species of 

 Balanoglossus, for in one case (Tornaria from 

 the Bahamas) he even refers them to scattered 

 mesenchyma cells of the primary body cavity. 

 Also the origin of the so-called heart-sac 

 is not yet sufficiently clear. Morgan would 

 refer it likewise to an accumulation of mesen- 

 chyma cells, whereas Spengel adheres to his 

 former account of the ectodermal origin of 

 this organ. Moreover, it seems to be im- 

 possible to re«oncile the new results with the 

 earlier account, and for this reason we must 

 refer to the original articles. — K.J 



The earliest fundaments of the genital 

 organs occur as pyriform sacs, and are found 

 in close connection with the ectoderm, a fact 



air 



Fig. 171.— Stage of de- 

 velopment of Balanoglossus 

 (after Agassiz, from Bal- 

 four's Comparative Embryo- 

 logy), an, anus; hr, gill- 

 pockets ; ht, "heart"; m, 

 mouth ; W, the so-called 

 water-vascular vesicle. 



1 Spengel, however, affirms that the " heart " is formed as a thicken- 

 ing of the epidermis next the acorn pore ; but perhaps this statement 

 can be harmonized with the opinion expressed above by assuming that 

 in this case the development of the so-called heart took place later, that 

 is to say, when the lining of the internal cavity of the acorn by means 

 of the water-vascular vesicle had already been accomplished. This, 

 however, is only conjecture. 



