318 EMBRYONIC DEVELOPMENT 



tuated by two forces, thematerial 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.* 



The material law, having predominated in the above cases, it gave origin to an antici- 

 pated development, which went 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 

 80). The embryo now elongates, becomes thinner, whilst its back rises (figs. 84, 86 

 and 87). Sometimes the anterior region is likewise divided beneath into two halves 

 by the prolongation of the longitudinal groove, so that the entire lower portion of the 

 body, when seen from below, exhibits four hillock-like elevations (fig. 85). The trans- 

 versal groove now deepens and the back rises more and more (figs, 82, 83 and 89), 

 until after a few days the embryo escapes and passes into the next period of its his- 

 tory, which is that of the larva. 



* 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. 



