204 INTERNAL FACTORS IV. i 



T. H. MORGAN. A study of a variation in cleavage, Arch. Ent. Mech. 

 ii, 1896. 



T. H. MORGAN. The formation of one embryo from two blastulae, 

 Arch. Ent. Mech. ii, 1896. 



L. SALA. Experimented Untersuchungen iiber die Reifung und 

 Befruchtung der Eier bei Ascaris megalocephala, Arch, miki: Anat. 

 xliv, 1895. 



0. L. ZUR STRASSEN. Ueber die Riesenbildung bei ^Urom-Eiern, 

 Arch. Ent. Mech. vii, 1898. 



H. E. ZIEGLER. Ueber Furchung unter Pressung, Verh. Anat. Gesell., 

 1894. 



R. ZOJA. Sullo sviluppo dei blastomeri isolati dalle uova di alcune 

 meduse (e di altri organismi), Arch. Ent. Mech. ii, 1896. 



7. NEMERTINEA. 



In the behaviour of its isolated blastorneres the Nemertine 

 egg resembles the Echinoderm very closely; the cells segment 

 as parts but ultimately give rise to wholes. The limit of this 

 capacity is, however, sooner reached, for there is a sharper 

 distinction between the potentialities of animal and vegetative 

 blastomeres. 



Experiments have been made on Cerebralnlus lactens by Wilson, 

 on Cerelratulus marginaius by Zeleny ; Zeleny and Yatsu have 

 further investigated the development of fragments removed from 

 various regions of the egg. 



The axis of the egg is marked by the excentricity of the nucleus 

 and the point of extrusion of the polar bodies. Maturation 

 precedes fertilization. The first two furrows are meridional 

 and divide the egg into four equal cells (A, B } C, and I)}. 

 Subsequent cleavage is, however, on the spiral plan seen in Tur- 

 bellarian, Annelidan, and holoblastic Molluscan eggs. 



In the Nemertine the egg-axis becomes the axis of the Pilidium 

 larva, the animal being the aboral, the vegetative the oral, pole. 



In both the ^ and the blastomeres cleavage is partial and an 

 open blastula is formed, or sometimes a flat plate of cells merely, 

 sometimes a closed sphere. The \ later becomes a larva with an 

 apical organ, mesenchyme and archenteron, but no lappets ; the \ 

 larva has a solid archenteron but neither apical organ nor lappets. 

 Development is slightly retarded; \ blastomeres give rise to a 



