382 SUMMARY OF CTJERENT RESEARCHES RELATING TO 



float freely, being driven, when mature, into two sexual pouches at the 

 anterior end of the body. 



In about fifteen minutes after fertilization two polar globules 

 are protruded from the egg, exhibiting a rhythm precisely similar to 

 that of the segmenting ova. Segmentation is, exceptionally among 

 Annelids, perfectly regular and uniform. About the 6th hour a gas- 

 trula is formed by a typical invagination, and at the same time the 

 region opposite the blastopore becomes marked off as the anterior 

 extremity and already functions as a head. A preoral band of cilia 

 appears and is subsequently replaced by a row of longer and 

 more powerful cilia. The transformation of the gastrula into a 

 trochosphere larva takes place by a peculiar method of growth 

 whereby the direction of the long axis is changed. The mesoderm 

 has a dual origin resulting in two different systems. First there is 

 formed the two mesodermal bands so common to Annelid larvae, 

 and the second part of the mesoderm consists of a large number of 

 unicellular muscles that separate from the endoderm at the time of 

 the invagination, having thus an origin very similar to that of the 

 mesoderm in Echinoderms. 



Three other ciliated bands soon make their appearance, one 

 immediately behind the mouth, a second just in front of the anus, 

 and a third is found upon the ventral median line in precisely the 

 place where the ventral nerve-chain is to arise. It is thus seen 

 that both the cerebral ganglion and the ventral nerve-chain are 

 preceded by the development of cilia from the very cells from 

 which the nervous elements are to arise, an interesting point as 

 indicating that already these cells are differentiated as nervous 

 elements, although at first there is no trace of any nervous system. 

 The further changes observed were the segmentation of the meso- 

 dermal bands and the origination of the ventral nerve-cord from the 

 ectoderm as a bilateral structure. 



Spermatogenesis and Fecundation in Ascaris megalocephala.* — 

 P. Hallez finds that the spermatospores of Ascaris megalocephala 

 are at first formed of a homogeneous, extremely transparent, and 

 nucleated protoplasm. Increasing in size they give rise by division 

 of the nucleus to four protospermatoblasts which become sepa- 

 rate. These similarly produce a second generation of cells — the 

 deutospermatoblasts, and have a central blastophore in young, 

 though not in old, males. When the deutospermatoblasts attain to 

 a size of 6 /x in diameter their protoplasm, which was before 

 homogeneous, becomes finely granular. When they have a diameter 

 of about 18 fji they divide into two, and henceforward their protoplasm 

 is filled with refractive granules. Before they pass into the seminal 

 vesicle the spherical cells conjugate by pairs, and the nuclei fuse with 

 one another. Two cells again separate, and at this moment cor- 

 puscles like polar globules are to be observed ; these, which the author 

 calls waste-corpuscles (corpuscles de rebut) finally entirely disappear. 



The deutospermatoblasts are then introduced into the organs of 



* Comptes Eendus, xcviii. (1884) pp. 695-7. 



