70 Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 
cylindrical, and is separated anteriorly from the ventricle by a slight constriction. Above the anterior 
portion of the ventricle and bulbus cordis is seen the dilated atrium (Ai.). The liver (L.) now extends 
posteriorly to the tenth turn of the spiral valve. The right and left lobes are of almost equal size, and 
the median mass which apparently arises from the left lobe is much increased in size as compared with 
the preceding dissection. The yolk stalk (Y.s.) lies in a notch between the median and the right lobe. 
In its cut end are seen in cross section the vitelline duct, the vitelline artery and the vitelline vein. The 
ventral lobe or head pancreas (Panc.) and the pancreatic duct have the same position as that occupied in 
preceding dissections. The sub-intestinal vein (V.sub-int.) forms a prominent ridge extending from the 
pancreas backward nearly to the last turn of the intestine. The intestine is decidedly constricted anterior 
to the digitiform gland. This is associated with changes in structure which are described in the tables on 
page 37. The digitiform gland (Dig.yl.) now lies posterior to the intestine. The cloacal plate (Cl.p.) is 
diamond-shaped in outline and bounded by a pair of low rounded folds. 
A comparison of the members of the normal plate series with some stages established 
by other authors. 
The literature of elasmobranch embryology contains a number of important contributions in which 
the embryonic period is considered as divided into a series of stages or phases. Broadly speaking these 
contributions may be separated into two classes. First, those containing descriptions of certain individual 
specimens which are considered as norms by which the position of other specimens, in relation to their 
development, may be determined. Such “norms” have generally received some symbol such as a letter or 
number, and this symbol also designates the stage they represent. A second class contains papers presenting 
divisions of the embryonic period based upon general characters of external form or internal structure. 
Any division into formal stages of a period of gradual change and differentiation is perhaps open to 
criticism as being arbitrary and artificial. The stages established by the method of the first class of con- 
tributions mentioned above have the advantage of precision. They are applicable however only to the 
species they represent or to closely related forms. In avoiding this disadvantage the stages presented in 
contributions of the second class are generally characterized in terms so general that the exact placing of 
any embryo is often very difficult. | 
Artificial as the device of formal stages may be it has proved sufficiently useful to become thoroughly 
incorporated in the literature of elasmobranch development, and it has been thought desirable to attempt 
to correlate the series of embryos described in the preceding pages with some of the well known series 
and stages of other authors. The results of this attempt are embodied in a general correlation table which 
is to be found on p. 72 of this paper. To make this table more intelligible there follow below brief reviews 
of the contributions which have been so tabulated. As the stages presented in the classic work of BALFOUR 
are so frequently referred to in elasmobranch work they are considered in some detail, and when an author 
has correlated his series with this well known one I have given the correlation in tabular form. 
In addition to these papers there are included in the table and the discussion several contributions 
in which no attempt is made to present formal stages but which contain descriptions of at least part of 
the anatomy and sometimes of the external appearance of series of elasmobranch embryos. Most of these 
papers deal with Squalus acanthias. 
