184 bulletin: museum of comparative zoology. 



cephalad for a very short distance only. In another series of the same 

 length of incubation the abducens was not found. It will be noted that 

 the abducens appears several hours after the differentiation of the funda- 

 ment of its muscle, the posterior rectus. 



In the ninety-three-hours' series, the nidulus of the abducens can be 

 seen lying close to the ventral surface of the hind-brain wall. It is 

 elongated in a longitudinal direction, and lies about 175 micra from the 

 median plane. It is not possible, in my preparations, to make out its 

 limits as definitely as in the case of the oculomotor nidulus. 



The nerve itself is a slender one, springing from the ventral face of 

 the hind- brain by a varying number of attenuated roots, placed one 

 behind the other, and crowded with " accompanying" cells. As many 

 as eight roots have been counted, the number differing in different em- 

 bryos, and even on opposites sides of the body in the same embryo. 

 The roots unite a short distance from the brain to form the trunk of the 

 nerve, which passes straight cephalad, running parallel to the ventral 

 face of the hind-brain. In vom Rath preparations the nerve can easily 

 be traced as a slender bundle of a few darkly stained neuraxons with 

 elongated " accompanying " cells to the posterior edge of the compact 

 cluster of cells making up the fundament of the posterior rectus muscle. 

 It will thus be seen that the abducens, though appearing later than the 

 oculomotor, is the first of the two nerves to become connected with a 

 muscle fundament. The fundament' of this muscle, the posterior rectus, 

 ■was already recognizable in the mesenchyme of Stage I, therefore, long 

 before any of the other eye-muscle fundaments, none of which can be 

 made out previous to the present stage. 



4. Eye Muscles. The fundament of the posterior rectus eye muscle 

 lies near the anterior portion of the hind-brain in a ventro-lateral posi- 

 tion. It consists of a mass of modified mesodermal cells, which differ 

 from surrounding ones in their closer association and their rather greater 

 amount of cytoplasm. The absence of fibres in the muscle fundament, 

 and their presence in that of the ciliary ganglion, make a notable differ- 

 ence in the appearance of these two structures. The muscle mass is 

 elongated in an antero-posterior direction (Plate 7, Fig. 23, mu. rt. p.). 

 It lies mostly caudad of the fundament of the ciliary ganglion, but its 

 anterior end runs cephalad and laterad as a narrow prolongation, which, 

 passing laterad of the posterior half of the ganglionic fundament, termi- 

 nates between it and the anterior cardinal vein. The fundament of this 

 muscle appears in Plate 7, Figure 24 {mu. rt. p.), where the section has 

 passed very near its anterior extremity. 



