ZOOLOGY AND EOTANY, MICROSCOPY, ETC. 37 



strating this point, and, in describing their later history, states that 

 they at first so alter their position that the right one comes to lie 

 almost in the middle of the neck ; the left approaches towards the 

 lower side, but does not come so near the middle line. This change 

 in position is to be explained by the torsion of the anterior part of the 

 visceral sac. In time the right ansa becomes altogether lost, while 

 the left is finally an aggregate of ciliated cells, in which irregular 

 cavities may be made out, and which are perhaps due to an internal 

 destruction of the cells. 



The liver is shown to be developed from the lower cells of the 

 blastula, and from those which became invaginated to form the gastrula, 

 and afterwards formed a solid sphere in the pseudo-gastrula. In- 

 vagination gave rise to two unequal portions, of which one remained 

 solid, while the other was excavated and filled with fluid. The author 

 finds that the whole of the enteric canal derives its elements from 

 the ectoderm, while the liver is directly derived from the endoderm, 

 its cells retaining throughout the whole of embryonic life the characters 

 of true endodermal cells, distinguished by drops of deutolecithin ; 

 these last increase in size with the cells themselves, and filling them 

 up, press the nucleus and protoplasm towards the outer wall ; the more 

 they aggregate in the cells, the more mucous substance is collected 

 in the cavity inclosed by the hepatic sac. The two lobes of the liver, 

 which are at first almost spherical, elongate considerably with the 

 growth of the embryo. 



The author is not able to demonstrate, though he is convinced of, 

 the completely medioventral position of the developing enteron ; at 

 first the rudimentary intestine consists of nothing but a collection of 

 ectodermal cells ; later on a cavity is developed in them, and this 

 pretty rapidly extends forwards and backwards ; thus we get a 

 tube bent at an oblique angle in the middle, and attached at either 

 end. A little later, the fore-gut bends towards the left, and the hind- 

 gut to the right side of the embryo ; the former follows the hej)atic 

 lobes, just as these follow the visceral nucleus and all the organs. 

 This torsion is easily explained on purely mechanical principles ; an 

 elastic cord growing regularly in length will, if its ends be fixed, form 

 a loop by torsion through 180° ; and thus tensions are avoided, which 

 would otherwise affect it. This law is called the law of torsion, and 

 would appear to be applicable not only to the phenomena observed in 

 Gasteropods, but also in other animals, in no way closely allied to 

 them. The body of the loop is formed by the widened portion of the 

 hind-gut, which will become the stomach, and this portion is so 

 twisted round that the anus comes to be near to, instead of at the 

 opposite pole to, the mouth. A similar change in position is effected 

 by the shell-gland. 



The earliest rudiments of the nervous system are seen in what is 

 now generally called the trochosphere stage. The following are the 

 leading peculiarities of this stage in Biihynia : — The body furms a 

 slight swelling towards the left side, the herald of the torsion ; the 

 intestine is slightly bent forwards, and lies towards the right hand ; 

 the fore-gut is hollowed out, and communicates with the buccal 



