106 



SCIENCE. 



[N. S. Vol. TII. No. 160. 



vestiges or ancestral reminiscences of such ori- 

 gin. 



A re-examination of the cell-lineage of a poly- 

 clade, Leptoplana, shows that, as in the annelid 

 or gasteropod, all of the first three quartets of 

 micromeres give rise to ectoblast, while the 

 second quartet gives rise also to mesoblast, each 

 cell of this quartet segmenting off three ecto- 

 blast-cells and then delaminating a large meso- 

 blast-cell into the interior. The third quartet 

 apparently gives rise to ectoblast alone, though 

 the possibility of its producing mesoblast is not 

 excluded. The four macromeres remaining 

 give rise to the archenteron, as Lang describes, 

 first dividing to form four basal cells (corre- 

 sponding in origin and position with the four 

 basal entomeres of annelids and mollusks) and 

 four much larger upper cells which correspond 

 to the fourth quartet of micromeres in annelids 

 and mollusks. The posterior of these cells al- 

 ways divides before the others, sometimes equal- 

 ly and symmetrically, as in Discoccelis (Lang), but 

 more often unequally. The cells thus formed 

 give rise to a part of the archenteron and not, 

 so far as can be determined, to mesoblast. 



These observations show that the mesoblast 

 of polyclades is of ectoblastic origin, and they 

 suggest that the origin of mesenchyme-cells 

 from the second {Unio, Crepidula) or third 

 (Physa, Planorbis) quartets in gasteropods may 

 be a vestige or ancestral reminiscence of the 

 mesoblast formation in the polyclades. They 

 suggest, further, that the mesoblast-bands 

 (entomesoblast) of annelids and mollusks may 

 have been historically of later origin than the 

 mesenchyme (ectomesoblast) — a view which 

 harmonizes, broadly speaking, with that of 

 Meyer —and that the two symmetrical entoblast- 

 cells, into which the posterior member of the 

 fourth quartet divides in the polyclade may rep- 

 resent the prototypes of the entomesoblasts of 

 the annelids and gasteropods. 



Mr. Crampton briefly reviewed his observa- 

 tions on the early history of the egg in Molgula 

 manhattensis, as follows : The author empha- 

 sized the fact that development begins not with 

 the cleavage or fertilization processes, but even 

 before. From the origin of the primary oocyte 

 until the final assumption of the adult form, 

 there is a continuous series of developmental 



changes, each stage being based upon the pre- 

 ceding one and conditioned by it. 



The growth of the primary oocyte and the 

 formation of the yolk were considered at some 

 length. A true ' yolk-nucleus ' arises, as the 

 author believes, from the nucleus, and this by 

 continued growth, and later by fragmentation,, 

 gives rise to very small spherules which later, 

 by enlarging, form the yolk-spherules. The 

 yolk-nucleus is an albuminous body closely al- 

 lied to, if not identical with, the yolk or deuto- 

 plasm. This was indicated by a large number 

 of microchemical tests. The yolk-nucleus at a 

 very early stage of the egg was also shown to 

 be the only albuminous body in the cell, for 

 the rest of the extra-nuclear part of the cell is 

 almost exclusively composed of pseudo-nucleinic 

 substances. Evidence was cited which indicated 

 that the yolk-nucleus was formed by the nucleus, 

 and that it enlarged by constant additions to it 

 from the nucleus. 



The more important results of a study of the 

 maturation and fertilization processes might be 

 briefly stated, although a fuller account will ap- 

 pear in the published paper. The first matura- 

 tion spindle arises entirely from the germinal 

 vesicle. It is peculiar in that it is barrel -shaped 

 and does not, as far as can be determined, bear 

 at either end centrosomes or asters. The first 

 polar-body receives sixteen chromosomes, while 

 sixteen remain in the egg. The second matura- 

 tion spindle is also barrel-shaped and is also 

 devoid of controsomes and asters. Eight chromo- 

 somes remain in the egg. The sperm entrance 

 was described and evidence was brought for- 

 ward to show that the centrosomes of the first 

 cleavage figure were derived from the sperm. 



The spindle of the first cleayage figure ap- 

 pears to be formed from the segmentation 

 nucleus, there being no ' central spindle' ex- 

 tending between the centrosomes. The spindle 

 itself was shown to be barrel shaped, the 

 daughter chromosomes reforming into a 

 vesicular nucleus at the ends or heads of the 

 barrel. A ' zwischen-korper' also arises, as in 

 the maturation stages, by a concentration of 

 the spindle fibres at the equator of the figure. 

 After the reformation of the daughter nuclei, 

 and after division of the cell-body, the paired 

 daughter centrosomes and asters diverge. The 



