January 21, 1898.] 



SCIENCE. 



105 



rounded by large yolk-spheres. As the egg 

 gradually fills with yolk the cytoplasmic radia- 

 tions become less conspicuous and shorter. 

 Finally, the egg-cell becomes filled with yolk- 

 spheres, except a small region about the vascu- 

 lar loop, which contains a reticular cytoplasm, 

 from which fine threads pass out among the 

 yolk-spheres. The nucleus is at the opposite 

 •end of the oval egg and is almost surrounded 

 by yolk. 



Now the vascular loop disappears ; the egg, 

 surrounded by its membrane, becomes detached 

 from its pedicle, and the point of attachment 

 becomes the micropyle, lying just over the small 

 cytoplasmic area. 



The temporary radiate arrangement of the 

 ■cytoplasm, resembling in many respects an 

 .aster, is apparently closely connected with the 

 deposition of yolk, and with the fact that the 

 •egg receives its nutriment from one end, i. e. , 

 from the vascular loop. 



CM. Child. 



Observations on the Cytogeny of Annelids with 

 ^ Equal' Cleavage. — Podarke obscura. The seg- 

 mentation is of the so-called ' equal' type. 

 At the 8-cell stage all the cells are equal 

 in size, and up to the 56-cell stage 

 there are no differences between the quad- 

 rants. Five groups of ' micromeres' are 

 formed, which have the same history as the 

 <!orrespondiug groups described for other 

 annelids, but the cells d^ (x) and d* (m) are no 

 larger than the other cells of the corresponding 

 quadrants. The median plane of the embryo 

 forms an angle of approximately 45° with the 

 first two cleavage planes. 



Bilateral symmetry in the embryo is pro- 

 duced by the formation of a small cell 

 {x'-^.) at the 64-cell stage ; by the bilateral di- 

 vision of one of the 4th group of micromeres to 

 form the mesoblast, and by the appearance of 

 & bilateral cross at the upper pole, these two 

 latter divisions occurring immediately after the 

 •64-cell stage. From now on the original radial 

 symmetry is rapidly lost. 



A few observations on other annelids follow. 



Lepidonotus sp. Here are formed the regular 

 number (5) of groups of micromeres, the cell 

 x'-^-, and the bilaterally symmetrical cross. 



Sthenolais picta. The cross furrow takes and 

 retains the position characteristic of the other 

 annelids, and the cell x'-'- is formed at the 

 usual time and place. 



Hydroides dianthus. Five groups of micro- 

 meres appear and one of the 4th group divides 

 bilaterally at the surface (mesoderm?). The 

 primary trochoblasts divide but once, thus 

 forming a primary prototroch of 8 cells instead 

 of 16, as in other annelids. 



A. L. Teeadwell. 



In addition to the above papers the following 

 reviews of recent literature were given during 

 the month : ' The Yolk-Nucleins in Birds and 

 Mammals' (Mertens), F. L. Charles; 'The 

 Correspondence in the History of the Germ 

 Cells in Plants and Animals' (Hacker), Miss 

 M. M. Sturges ; ' The Development of the 

 Excretory System of the Myxinoids' (Mass), 

 Miss E. R. Gregory. 



NEW YORK ACADEMY OF SCIENCES— SECTION OF 

 BIOLOGY, DECEMBEE 13. 



In a paper entitled ' Considerations on Cell- 

 Lineage, based on a Re-examination of some 

 Points on the Development of Annelids and 

 Polyclades,' Professor E. B. Wilson presented 

 observations regarding the origin and relations 

 of the mesoblast in annelids and polyclades 

 which illustrate the fact of ancestral reminis- 

 cence in cell-lineage. In some of the Annelids 

 (Aricia, Spio, Nereis and others) the primary 

 mesoblasts have not been properly so-called ; 

 for before giving rise to the mesoblast-bands 

 they bud forth cells that may be, in some cases, 

 traced into the wall of the archenteron. In 

 Nereis not less than six or eight such cells are 

 formed ; these become pigmented, wander into 

 the interior, and finally give rise to the posterior 

 part of the archenteron. In Aricia and Spio 

 only a single pair of corresponding cells is 

 formed, and they are so small as to play a quite 

 insignificant part in the building of the body. 

 A comparison of these results with those of 

 Conklin on Crepidula indicates that the meso- 

 blastic pole-cells of annelids and mollusks are 

 to be regarded both historically and ontogenet- 

 Icalljf as derivatives of the archenteron, and that 

 the rudimentary cells of Aricia and Spio are 



