No. 2.] THE MESO BLAST-BANDS IN ANNELIDS. 215 



This is precisely what occurs in Nereis. The term "ectoblast" 

 as applied to the ventral plate as a whole is, however, a misno- 

 mer. Its cells form rather a still undifferentiated tissue which 

 is nowise to be regarded in the same light as the ectoblast of 

 the upper pole. 



II. 



It now becomes an interesting question whether the second- 

 ary mesoblast of annelids can be shown in all cases to arise 

 from a single pair of teloblasts ; for the case of Nereis shows 

 that they may be present only in very early stages, so as easily 

 to be overlooked. The entire subject demands re-investigation 

 from this point of view. I have carefully studied the develop- 

 ment of Hydroides diantJms (a form nearly allied to Eupomatus), 

 by following the cleavage of the living ovum, by examination of 

 stained and cleared embryos, and by actual sections. The 

 cleavage is in every detail identical with that of Eupomatus, the 

 gastrulation takes place in essentially the same manner, and the 

 trochophore is of quite the same type. Yet I have been unable 

 to identify the teloblasts at any period. They are certainly not 

 present at a stage when the mesoblast-bands consist of not more 

 than five or six cells each {cf. Hatschek's Figs. 43 to 46).^ At 

 this period each band ends posteriorly in a group of about three 

 cells, two of which, not perceptibly larger than the others, are 

 joined by a narrow bridge of protoplasm stretching across in the 

 angle between the proctodaeum and the wall of the anal vesicle. 

 The head-kidney lies outside the mesoblast-band, and is only 

 connected with it at its anterior end.^ 



In its earliest recognizable condition (cf. Hatschek's Fig. 33) 

 the mesoblast-band consists of a group of three or four cells 



1 Arb. a. d. Zool. Inst. Wien., VI., 1885. 



2 I have been able, by the study of these embryos, to establish the interesting fact 

 that the head-kidney opens posteriorly into the proctodaeum. Under a high power 

 (Zeiss Apochromatic Homogeneous, 3 mm.) the canal can easily be followed from 

 its beginning near the front end of the organ, along the outer dorsal border of the 

 latter, into the antero-lateral part of the proctodaeum. This can be done most 

 readily during the activity of the cilia (which is intermittent) ; but the canal can 

 easily be followed in the quiescent state, in both the side and ventral views of the 

 larva. This fact seems to remove all doul^t of the homology of the trochophoran 

 head-kidney with the nephridia of the Rotifera. I shall return to this point at 

 another time. 



