396 SUMMARY OF CUEKENT llESEARCHES RELATING TO 



matter in the form of dark specks and elongate rods is formed during 

 this process, and constitutes the characteristic pigment of the late 

 embryos and larvte. The ventral thickening of the ectoderm is found 

 at an early stage. The endoderm appears under the ectoderm, first as a 

 number of cells at the anterior end, and ultimately as a complete chain 

 of cells extending over the inner yolk, except at the point of prolifera- 

 tion at the posterior end. The cells occupying the blastocele break 

 down like the outer cells, and become a mass of yolk-granules, in which 

 are scattered a number of nuclei with associated protoplasm. Some of 

 these pass outwards to form the endoderm, some pass inwards to form 

 vitellophags. A lumen is formed in the yolk-mass, and it becomes 

 connected to the posterior involution, giving rise to a gastrula-like 

 structure. The internal mass of yolk assumes the form of a number of 

 yolk columns or pyramids. The posterior body-cavities arise by a 

 number of cells from the yolk-mass forming a second layer under the 

 endodermal layer. The yolk lumen increases in size, the yolk-granules 

 becoming converted into a homogeneous substance. This takes place 

 chiefly on the dorsal side, where the yolk-lumen becomes connected 

 with the archenteron. The position and extent of the five body 

 cavities in the larva are shown. The yolk in the larva is in the form of 

 an elongate mass of granules and homogeneous matter Iving on the floor 

 of the archenteron. Changes are described in the larval nervous 

 system, and the appearance of a posterior thickening and involution of 

 the ectoderm below the yolk-mass is noted. 



Phoronis ovalis.* — S. F. Harmer found that the eroded appearance 

 of a shell of Neptunea antiqua was due to the fact that the substance of 

 the shell was traversed by the burrows of boring animals, most of which 

 belonged to a minute species of Phoronis corresponding closely with the 

 P. ovalis of 'Strethill Wright, which has usually Ijeen regarded as the 

 immature form of some other species. It is now shown to be a well- 

 characterized adult form inhabiting burrows which it excavates in the 

 shells of molluscs. It possesses in a high degree the faculty of regener- 

 ating the distal end, which is of common occurrence in the genus. Its 

 gregarious habit is probably the result of its power of reproducing by 

 transverse fission, a process which takes place repeatedly and profusely. 

 There is reason to believe that a similar process occurs in certain other 

 species which are found as colonies consisting of numerous individuals, 

 though it is uncertain whether these have the power of reproducing by 

 fission. 



Echinoderma. 



Permeability and Activation of Sea-Urchin Eggs.f — Ealph S. Lillie 

 has experimented with the eggs of Arbacia with reference to the rate of 

 entrance of water, when the eggs are fertilized or unfertilized. His 

 previous work showed that in dilute sea-water fertilized eggs take up 



* Quart. Jouru. Micr. Sci., ii. (1917) pp. 115-48 (3 pis.). 

 t Amer. Journ. Physiol., xliii. (1917) pp. 43-57. 



