OF PLANOCERA ELLIPTICA. 
19 
elasticity by which the primitive spherical form is subjected to the most diversified 
though temporary variations. This fact is another evidence that cells may contract as 
well as muscular tissues, although the physiology of this function is not easily explain- 
ed in the actual state of our knowledge on this subject.* 
§ 5. The transformation of the spherical embryo into a symmetrical animal is effec- 
tuated by two forces, the material force and the organic force, by which a different 
development follows as either the one or the other of these forces prevails. 
A. The embryonic sphere, being subjected to a rotatory motion around an axis, 
may elongate and assume an oblong shape (fig. 57), such as would be originated under 
the mathematical or astronomical law. Then one of the regions acquires a prepon- 
derance over the other, the embryo moves forwards, thus indicating a cephalic region. 
From this instant there are two sides, a right and a left (fig. 59 and 61) : the body 
being flattened or depressed, very much resembling that of the parent. There are 
two eye specks upon the anterior region, the only organs as yet conspicuous. The 
little animal crawls about when caused to escape prematurely out of the egg’s external 
membrane. Some individuals are provided with a caudal, others with a cephalic 
needle-like appendage, varying in length and more transparent than the body, which 
is provided with vibrillae all around. The external layer of the body is also more 
transparent than the general mass (fig. 60 and 70). The same embryo may alternately 
assume the forms represented in figs. 66 — 68 and 70 — 74. The surface of the body 
is incessantly waving or undulating under the constant agitation of the centre.t 
The material law, having predominated in thejabove cases, it gave origin to an antici- 
pated development, which w r ent on so rapidly as not to allow all metamorphoses to take 
their natural course; forms like those of the parents made their appearances within 
the egg’s envelope. 
The query now is, whether or not these embryos would live and grow into adult 
and perfect animals, duly representing their species, and capable of reproducing it? 
I may be mistaken, but I cannot help thinking that most of them would not survive, 
and that in their polymorphic and ever changing aspect we have the expression of a 
convulsive state of these organisms about to die out, a final struggle of a vital essence 
enveloped in an abnormal material body. 
B. But now let us see what forms the organic law of development brings about. 
The first modification which the embryonic sphere undergoes, is a quadrangular shape 
(fig. 75), which somewhat elongates (fig. 76), when a transversal groove is formed 
below, dividing an anterior from a posterior region (fig. 78). Then a longitudinal 
groove appears, which divides the posterior region into two halves (figs. 77, 79 and 
* I allude to the contraction of cells in jelly fishes (Medusae) observed by Prof. Agassiz, 
t In figs* 60— 67 the embryos are represented not surrounded by the outer membrane of the egg. Indeed, they 
had not escaped naturally, but were forced out by pressure before hatching. 
6 
