378 SUMMARY OF CURRENT RESEARCHES RELATING TO 



portion. It is sometimes thicker, sometimes narrower. The rounding- 

 off of the posterior part of the head is characteristic of the spermatozoa 

 of gorilla and man. The connecting portion is about as long as the 

 head ; it sometimes appears granular ; there is evidence of a spiral fibre 

 enclosing a straight rod. 



Nucleolar Changes during* Oogenesis.* — Jozef Nusbaum has studied 

 the various changes in the nucleolus during the oogenesis in certain 

 deep-sea fishes — Argijropehcus hemigijmnus and Sternoptyx diaijliana. 

 The nucleolus and the structures into which it is transformed must be 

 regarded as almost pure chromatin. The nucleolus is a quite inde- 

 pendent structure, but it is probable that in the stage of disintegra- 

 tion of nucleolus and nucleus — when the chromatin of both becomes 

 like dust — there comes about some mingling, so that there is some 

 chromatin from both in the reconstructed chromosomes. The produc- 

 tion of thread-like and coil-like structures from the nucleolus is described, 

 and it is shown that this is not due to any " vacuolization " of the 

 nucleolus. 



Mesoderm-formation in Megalobatrachus.f — Dan de Lange, jun., 

 distinguishes four regions of mesoderm-formation. 1. The anterior 

 head-mesoderm arises independently of the dorsal plate as a more or 

 less paired outgrowth of the roof of the archenteron. From it arises 

 the whole of the head-mesenchyme in front of the tip of the notochord. 

 It becomes divided into a dorsal portion (the third somite of Van 

 Wijhe ?) and two ventral portions (the pre-mandibular and the man- 

 dibalar). The dorsal portion unites with the parachordal mesoderm, 

 the mandibular mesoblast with the pericardium. 



2. The anterior unsegmented portion of the dorsal plate (proto- 

 chordal plate of Hubrecht ?) has a mixed origin. The sematogenic 

 dorsal plate projects into the head region, and there assimilates elements 

 of the archenteron-roof at its anterior margin. The latter remains for 

 a considerable time in connexion with the archenteron-roof. 



Thus is explained the mixed character of this portion of the meso- 

 derm. On the one hand, it forms typically somatic organs, such as the 

 notochord ; on the other hand, it forms typical cephalic organs, such 

 as the pericardium and the heart. Moreover, this region forms the 

 mesoderm of the branchial region behind the hyo-mandibular. 



3. The purely somatogenetic mesoderm is formed ])y inward prolifera- 

 tion of the dorsal plate at the somatopore margin, or by exaggerated 

 growth of the latter. The growing zone is circular, and the inward 

 proliferation at the dorsal side is much the more important factor. This 

 mesoderm portion is segmented, and from it arise the anterior twelve 

 somites of the trunk. 



4. The urogenetic mesoderm is formed, after the closure of the 

 somatopore, from a growing zone lying in front of the neurenteric canal. 

 It remains restricted to the dorsal median line. From this zone arise 

 the posterior twelve somites of the trunk and all the somites of the tail. 



* Anat. Anzeig., xliii. (1913) pp. 582-98 (1 pi. aud 11 figs.), 

 t Anat. Anzeig., xliii. (1913) pp. 250-79 (28 figs.). 



