372 SUMMARY OF CURRENT RESEARCHES RELATING TO 



In the history of development we see that segmentation is unequal, 

 and that the gastrula is invaginate ; the mesoblast is formed bilaterally 

 from the endoderm on either side of the blastopore ; from the time 

 when two or three mesoblast-cells are budded off on either side a 

 cavity is present in each mass so formed ; these cavities are the two 

 halves of the body-cavity, and the author regards the mode of origin 

 as a modification by simplification of the euterocoel type, as seen in 

 Argiope. The mesoblastic diverticulum into the pre-oral lobe grows 

 rapidly, and distinct somatic and splanchnic layers soon become 

 apparent ; the muscle-cells in this region have all the histological 

 character of the mesenchyme of the Hertwigs. The endoderm becomes 

 thickened in the pre-oral lobe to form the future nerve-ganglion, and 

 as a post-oral ring indicating the j)ositiou of the line of future ten- 

 tacles in the circumoesophageal nerve-ring of the adult. The anus 

 is, from the first, terminal in position, and the four divisions of the 

 alimentary canal early become apparent ; the cells of the first stomach, 

 though ciliated, are much more amoeboid than in the adult, and 

 throughout larval life digestion goes on in this region. The whole 

 mesoblast arises as two endodermic sacs, the walls of which form 

 somatic and splanchnic layers. 



The vessels were seen to arise as splits in the splanchnopleure. 



The author is of opinion that the life-history of Phoronis offers a 

 solution of many morphological problems ; the pre-oral ring, corre- 

 sponding to the velum of a Trochosphere, is from the earliest stages 

 reduced relatively to the post-oral ; the latter persists throughout 

 life as a circumoesoi^hageal ring ; the ganglion of the pre-oral lobe 

 disappears with the change from a free to a fixed mode of life. The 

 whole body-cavity is an enteroccel ; the intracellular portion of the 

 excretory system atrophies when the vascular system is developed. 

 The identity of the P/(oroH/s-larva, up to the formation of the 

 nephridia and before the outgrowth of the anal region, with the Tro- 

 chosphere-type of Hatschek is complete; the distinction drawn by 

 the brothers Hertwig between the histological characters of the mesen- 

 chyme and mesoderm utterly breaks down in Phoronis, and it may be 

 suggested that the other Trochospheres are enterocoeles. The author 

 discusses briefly the relations of Phoronis to the Brachiopoda and 

 Polyzoa, and suggests that the larvae of these forms are modified from 

 the Trochosphere by the earlier attainment of the relation of the 

 ventral surface which in Phoronis is only accomplished during the 

 metamorphosis. 



Development of Borlasia vivipara.* — W. Salensky finds that, 

 though the males are much smaller than the females, the generative 

 organs are in both sexes formed on the same type, being constituted 

 by paired sacs which open to the exterior by pores at the sides of the 

 body. These sacs are lined by a secretory epithelium, the ovum is 

 fertilized in situ, and there remains till the larva is developed. Seg- 

 mentation is complete and unequal, and before the blastula is con- 



* Bull. Sci. De'p. Nord, v. (18S2) pp. 462-9. 



