342 
E. W. MACBRIDE. 
38. Wilson and Hill, 
vliynclius,” ‘ 
1907. 
— “Observations on the Development of Ornitho- 
Pliil. Trans. Roy. Soc. Loud.,’ series B., vol. cxcix, 
EXPLANATION OF PLATES 18—21, 
Illustrating Prof. E. W. ^lacBride’s paper on “ The Formation 
of Layers in Aniphioxns.” 
[All the figures in this paper were drawm under the magnification 
afforded by a Zeiss apochromatic immersion objective of 2 mm. focal 
distance. The original magnification of 900 to 1000 diameters has l)een 
retained in the case of the smaller figiu-es, but the magnification has 
been reduced in the case of the larger to 600 diameters for convenience 
in i-eproduction.] 
List op Abbreviations. 
a. Anal diverticulum, a'. Upper section of archenteron. at. Rudi- 
ment of atrial ridge, hr.ect. Ectodermic portion of gill-pouch, hr.eud. 
Endoderinic portion of gill-pouch, ch. Notochord, chib. Club-shaped 
gland, coll. Collar-cavities. /. Ectodermic folds which extend over 
the nerve-plate. 7i. Karyokinesis (mitosis). l.coU. Left collar-cavity. 
l.h. Left head-cavity. 7.sp7. Left splanchnoccele. m. Place where the 
mouth will be formed, liiwsc. Muscular fibres, my. Myocoele. «.c. Neural 
canal. n.e. Neurenteric canal. nepli. Hatschek's nephridium. oc. 
Clear cells; first indication of eye-spot, r.coll. Right collar-cavity. 
r.h. Right head-cavity, r.spl. Right splanchnocele. tr. Trunk-cavity. 
V. Vacuole in spot where left head-cavity will acquire an opening to the 
exterior, x. Position of centre of growth which initiates gastriilation. 
y. Position of centre of growth which closes the blastopore. 
PLATE 18. 
All the sections on this plate are magnified 600 di.ameters. 
Fig. 1. — Median sagittal section through a flattened blastula in which 
the centre of growth at x is just making its appearance. 
Fig. 2 . — Median sagittal section through a slightly older embryo in 
wdiich gastrulation has just begun. 
Fig. 3. — Medial sagittal section through a still older einlnyo in which 
gastrulation has proceeded further. 
