RRMARKS ON THE DEVELOl'MENT OF AMPHIOXUS. 365 



EXPLANATION OF PLATE 17, 



Illustrating Mr. E. W. MacBride's paper entitled "Further 

 Remarks on the Development of Amphioxus." 



List of Abbreviations employed. 



ect. Ectodermal thickening of the atrial ridge, lung. muse. Longitudinal 

 muscles developed from the inner wall of the collar coelom, and constituting 

 the first myotome. mi/. The myotomes derived from the coelomic groove. 

 nch. The uotochord. sp.c. The spinal cord. t. muse. The transverse 

 muscle-fibres in the floor of the atrial cavity. 



The outlines of all the figures are drawn with a camera lucida, but in Figs. 

 5, 7, 9, and 10 the details of the histology of the ectoderm and endoderm are 

 not given. 



Fig. 1. — Transverse section through the anterior portion of a larva just 

 before the formation of the mouth, in order to show the continuity existing 

 between the first myotome and the collar coelom. Magnified 1100 diameters. 

 (Zeiss, oc. 2 ; Leitz, immersion jL.) 



Fig. 2. — Transverse section through the hinder end of the pharyngea 

 region of another larva of the same age as that from which Fig. 1 is taken. 

 The splanchnocoele (represented on the right side by a wedge of ceils) inter- 

 poses between the pharynx and the backward extension of the collar coelom. 

 Magnification as before. 



Fig. 3. — Another section from the same series as that to which Fig. 2 

 belongs, taken a little further back. On the right side the collar coelom has 

 disappeared, and the outline of the section has changed in consequence. 



Fig. 4. — Transverse section of the pharyngeal region of an older larva, in 

 which several gill-slits have been formed. The atrial ridges are not as yet 

 recognisable. Magnification as before. 



Fig. 5. — Transverse section of the pharyngeal region of a still older larva, 

 to show the first appearance of the atrial ridge. The metapleural coelom has 

 become separated from the remainder of the collar coelom. Magnification 350 

 diameters. (Zeiss, oc. 2, obj. D.) 



Fig. 6.— a small portion of another section from the same series as that to 

 which Fig. 5 belongs; taken further forward. It illustrates the peculiar 

 thickening of ectoderm cells on the atrial ridge. 



Fig. 7. — Transverse section of the pharyngeal region of a larva in which 

 the atrial ridges have just met, so as to form a floor for the atrial cavity. 

 ^Magnification 350 diameters. The metapleural coelom has become solid, and 



