ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



331 



showing three germinal layers, shows only the medullary groove and the 

 notochordal plate. The reality of a neurenteric canal in the embryo of 



Section through anterior part of embryonic shield of guinea-pig of 

 fourteen days. Magnified about 500. ect., ectoderm of medullary 

 groove ; s., a segment ; **, the two asterisks show where the inter- 

 vening notochordal plate stops and the endoderm recommences. 



Cavia is discussed, and the conclusion is that it does not exist. Finally, 

 the author gives an account of the origin of the amnion, emphasizing 

 its schizocoelic nature. J. A. T. 



Development of Pig's Sternum. — Frank Blair Hanson {Anat, 

 Record, 1919, 17, 1-23, 6 pis.). It is maintained that the pig's sternum 

 cannot be a derivative of the costal cartilages, since it exists as a distinct 

 entity prior to the fusion of the ribs with the sternum. It may arise 

 " in situ," as Whitehead and Waddell suggest, but more likely its genesis 

 is wrapped up in some way with the coracoidal hypothesis of Paterson. 

 Its primordium appears as two longitudinal bands, which become united 

 anteriorly by the presternum. The sternal bands are at first unseg- 

 mented ; the sternebrge and centres of ossification (double for each 

 sternebra) appear late in development, and are always intercostal in 

 position. The centre of ossification for the presternum appears last. 



J. A. T. 



Development of Pericardio-peritoneal Canals in Selachians. — 

 Edwin S. Goodrich {Joiirn. Anatomy, 1918, 53, 1-13, 18 figs.). An 

 examination of embryos of Acanthias and Scyllium confirms Balfour's 

 suggestion that the canal leading in the adult Selachii from the peri- 

 cardial to the peritoneal coelom, and opening into the latter by paired 

 apertures, is a remnant of the wide communication between these cavities 

 in the embryos. The canal openings are not new formations as Hoch- 

 stetter maintained, but are derived from the pericardio-peritoneal 

 passages above the mesocardia lateralia. A precise account of the 

 development is given. J. A. T. 



