George L. Streeter 141 
List of Embryos Reconstructed. 
| Length in mm. | Section. 
Number of | Probable age 
embryo. N. B. V.B. | ECE Thickness. Direction. 
148 4.3 | 3 20 10 uv Coronal. 
Bee LG 6.5 6.6 26 15 Sagittal. 
2 7 6 26 15 Coron-trans, 
163 9 | 9 30 2 Transverse, 
109 10.5 Di 33 20 Transverse. 
1% 13 lames 36 20 Transverse. 
144 12 | 14 37 40 Sagittal. 
22 18 20 44 50 Transverse. 
229 — 21 44 50 Sagittal. 
86 20 30 54° 50 Coronal. 
One or more wax plate reconstructions were made of each embryo 
after the method of Born. In most cases the models included the 
membranous labyrinth with the acoustic and facial nerves, and a portion 
of the central nervous system. Of these models seven were selected for 
illustration and are shown in Plates I and II. The form of the models 
has been controlled in all cases by dissections of pig embryos of cor- 
responding stages of development, prepared in the manner described in 
a previous paper (Streeter, 04, p. 87). Such: comparison was of 
particular assistance in the study of the nerves and ganglion masses. 
The value of these dissections was greatly increased by previously staining 
the embryos, in toto, with alum cochineal (powdered cochineal 6 gm., 
ammonia alum 6 gm., and distilled water 200 ce.), which produces a 
brilliant differentiation of the tissues. In the same way that a micro- 
scopical section is improved by staining so is a stained microscopical 
dissection that much better than an unstained one. In studying these 
a strong, direct illumination of the specimen is necessary. 
Whenever the size of an embryo is expressed by a single dimension it 
refers to its greatest length, and the age is that as determined by Mall’s 
rule, 7. ¢., age in days equals the square root of the greatest length times 
one hundred. 
The drawings for Plates I and II were prepared under the guidance 
and assistance of Mr. Max Brodel, for which the author derives pleasure 
in taking advantage of this opportunity to acknowledge his appreciation. 
MemBranous LABYRINTH. 
The auditory organ is generally described as developing phylogeneti- 
cally from the lateral line organs of the marine vertebrate, which sink 
beneath the surface of the body and develop a cartilagenous or bony 
