218 SUMMARY OF CURRENT RESEARCHES RELATING TO 



First Changes in Fecundated Ovum of Lepas.* — Frof. M. N ussbaum 

 finds that the processes of maturation and feeundation of tho ovum of 

 Lepas arrango the living parts in such a way that on tho extrusion of 

 the directive corpuscles all the axes of tho future animal arc already 

 defined. The position of the corpuscles indicates tho future position of 

 the cephalic end of the embryo ; tho first and second segmentations take 

 place along a plane which will be tho future long axis of tho animal. 

 If the relative position of the axis continued as at first, it might be 

 thought that the contents of tho ovum alone possessed the power of 

 orientation. But, as the first plane of division passes from a longitudinal 

 to an equatorial plane, the envelope and its form must also possess 

 directivo powers ; these may bo best explained by the principle of least 

 resistance. By this principle we may also explain the fact that the first 

 division, though it takes place in the longitudinal direction, does not 

 divide the ovum into the materials for tho right and left halves of the 

 body. The rigidity of the egg-capsule causes it to be tho essential 

 regulator of the position of the developing embryo of Lepas. 



Vermes, 

 a. Annelida. 



Development of Annelids.f — Prof. M. Salensky finds that three 

 stages of development play an important part in the evolution of worms. 

 They succeed one another in a definite order in the development of the 

 embryo, and consequently did so in the evolution of the phylum. They 

 may bo called tho Trochogastrula, the Trochophora, and tho Trocho- 

 neurula. 



Tho Trochogastrula represents a stage which is common to all worms, 

 and which serves as the genetic bond between the different classes of 

 this group ; it is in the form of a bilateral gastrula, the body of which 

 is divided into a preoral and a postoral portion, the former of which 

 contains the occipital plate. There is no anus ; the ciliated velum of 

 the gastrula is sometimes retained. 



The Trochophora represents a further stage, which is characterized 

 by the appearance of an anus and of a postoral ring, as well as by an 

 increase in the size of the postoral region of the body. 



The Trochoneurula is characterized by the development of medullary 

 plates. Comparative embryology shows that the different classes of 

 worms pass through one, two, or all three of these stages, and tho 

 classification of worms may, in consequence, be thus formulated : — 



(A) The Platodcs only pass through the Trochogastrula stage. 



(B) The Nemerteans and the Rotifers pass through the Trocho- 

 gastrula and the Trochophora stages. 



(C) The Annelida and Gephyrea pass, in addition, through the 

 Trochoneurula stage. 



The development of Nematohelminths presents enormous difficulties 

 to a comparison of their development with that of other classes of 

 worms, while they have no metabolic (larval) forms. Future researches 

 may throw further light on this problem. 



The author proposes to divide the worms into two groups, one of 

 which he calls Cephaloneura, and the other Neuraxonia. The former, 



* SB. K. Preuss Akad. Berlin, 1887, pp. 1052-5. Ann. and Mag. Nat. Hist., 

 i. (1888) pp. 101-2. 



t Arch, de Biol., vi. (1887) pp. 589-G53 (1 pi.). 



