PROMORPHOLOGICAL RELATIONS OF CLEAVAGE 379 



of the upper hemisphere represent the 'f animal layer," outer germ- 

 layer or ectoblast from which arise the epidermis, the nervous s)^tcm, 

 and the sense-organs. This fact, afterward confirmed in a very large 

 number of animals, led to the designation of the two poles as aniimil 

 and vegetative, formative and nutritive, or protoplasmic and dcuto- 

 plasmic, the latter terms referring to the fact that the nutritive deuto- 

 plasm is mainly stored in the lower hemisphere, and that development 

 is therefore more active in the upper. The polarity of the ovum is 

 accentuated by other correlated phenomena. In every case where 

 an egg-axis can be determined by the accumulation of deutoplasm in 

 the lower hemisphere the egg-nucleus sooner or later lies eccentri- 

 cally in the upper hemisphere, and the polar bodies are formed at the 

 upper pole. Even in cases where the deutoplasm is equally distrib- 

 uted or is wanting — if there really be such cases — an egg-axis is 

 still determined by the eccentricity of the nucleus and the corre- 

 sponding point at which the polar bodies are formed. 



In vastly the greater number of cases the polarity of the ovum has 

 a definite promorphological significance ; for the egg-axis shows a 

 definite and constant relation to the axes of the adult body. It 

 is a very general rule that the upper or ectodermic pole, as marked 

 by the position of the polar bodies, lies in the median plane at a point 

 which is afterward found to lie at or near the anterior end. Through- 

 out the annelids and mollusks, for example, the upper pole is the point 

 at which the cerebral ganglia are afterward formed ; and these 

 organs lie in the adult on the dorsal side near the anterior extremity. 

 This relation holds true for many of the Bilateralia, though the 

 primitive relation is often disguised by asymmetrical growth in the 

 later stages, such as occur in echinoderms. There is, however, some 

 reason to believe that it is not a universal rule. The recent observa- 

 tions of Castle ('96), which are in accordance with the earlier work of 

 Seeliger, show that in the tunicate Ciona the usual relation is reversed, 

 the polar bodies being formed at the vegetative {i.e. deutoplasmic or 

 entodermic) pole, which afterward becomes the dorsal side of the 

 larva. My own observations ('95) on the echinoderm-egg indicate 

 that here the primitive egg-axis has an entirely inconstant and casual 

 relation to the gastrula-axis. It may, however, still be possi]:)lc to 

 show that these exceptions are only apparent, and the principle in- 

 volved is too important to be accepted without further jDroof. 



{b) Axial Relations of the Primary Cleavage-planes. — Since the 

 egg-axis is definitely related to the embryonic axes, and since the 

 first two cleavage-planes pass through it, we may naturally look for a 

 definite relation between these planes and the embryonic axes ; and 

 if such a relation exists, then the first two or four bkistomeres must 

 likewise have a definite prospective value in the development. Such 



