268 
NATURE 
i _ Seki es 
| %uly 10, 1873 
2 A 
Gradually the nucleus divides itself, and the protoplasm 
also separates into two spherules (PI. 5, Fig. 12) ; these 
two subdivide into four (Pl. 5, Fig. 13), and so on (Pl. 5, 
Fig. 14), until at length thirty-two are present, compressed 
into a more or less polygonal form (Pl. 5, Fig. 15). Here 
this process ends. The separate spherules now begin to 
lose their smooth outline, to throw out processes, and 
to show amceboid movements like those of the crea- 
tures just described. The processes or pseudopods grow 
gradually longer, thinner, and more pointed. Their move- 
PLATE 6, 
Plate 6.—Figs. 1-4, Yolk-segmentation in Laomedea; 5—9, in Filaria; 
10—13, in Echinus ; 14—17, in Lacinularia ; 18—21, in Purpura ; 22—24, 
Amphioxus ; 25—29, Vertebrata. 
ments become more active, until at length they take the 
form of ciliz. The spherical Magosphcera, the upper sur- 
face of which has thus become covered with ciliz, now 
begins to rotate within the cyst or envelope, which at 
length gives way and sets free the contained sphere, which 
then swims about freely in the water (PI. 5, Fig. 16), thus 
closely resembling Synura, or one of the Volvocinex, 
After swimming about in this condition for a certain time 
the sphere breaks up into the separate cells of which it is 
composed (Pl. 5, Fig. 17). Aslong as the individual cells 
‘finally forming the young worm. 
had remained together they had undergone no changes of 
form, but they now show considerable contractility, and 
gradually alter their form, until they become undistinguish- 
able from true Amcebe (PI. 5, Fig. 18). Finally, according 
to Haeckel, these amceboid bodies, after living for a certain 
time in this condition, return to a state of rest, again con- 
tract into a spherical form, and secrete round themselves 
a structureless envelope. . , 
It may be said, and said truly, that the difference 
between such beings as these and the Campodea, 
or Tardigrade, is immense. But if it be considered 
incredible that even during the long lapse of geo- 
logical time such great changes should have taken place 
as are implied in the belief that there is any genetic con- 
nection between insects and these lower groups, let us — 
consider what happens under our eyes in the develop- 
ment of each one of these little creatures, in the pro- 
verbially short space of their individual life. 
I will take for instance the first stages, and for the sake 
of brevity only the first stages, of the life history of a 
Tardigrade.* As shown in Fig. 60, the egg is at first 
a round body, with a clear central cell—the germinal 
vesicle ; it increases in size, and after a while the yolk 
and the germinal vesicle divide into two (Fig. 61), then 
again into four (Fig. 62), and so on, just as we have seen 
to be the case in Magosphcera. From the minute cells 
(Fig. 63) arising through this process of yolk-segmenta- 
tion, the body of the Tardigrade is then built up. 
It is true that among the Insecta generally, normal 
yolk-segmentation does not occur, though the first stages’ 
of development in Platygaster, as figured by Ganin 
(ante Figs.), closely resemble those of the Tardigrada, 
Though I will not now attempt to point out the full 
bearing of these facts on the study of embryology generally, 
yet I cannot resist calling attention to the similarity of 
the development of Magosphcera with the first stages of 
development of other animals, because it appears to me 
to possess a significance, the importance of which it 
would be difficult to over-estimate. : 
Among the Zoophytes Prof. Allman thus describes Tt 
the process in Laomedea, as representing the Hydroids 
(Pl. 6, Fig. 1, represents the young egg) :—“ The first step 
observable in the segmentation-process is the cleavage of 
the yolk into two segments (Pl. 6, Fig. 2), immediately 
followed by the cleavage of these into other two, so that 
the vitellus is now composed of four cleavage spheres 
(Pl. 6, Fig. 3).” These spheres again divide (Pl. 6, Fig. 
4) and subdivide, thus at length forming minute cells, of 
which, as in the previous cases, the body of the embryo 
is built up. 
In Pl. 6, Figs. 5—9 represent the corresponding stagesin 
the development of a small parasitic worm—the A7/aria 
mustelarum—as given by Van Beneden.{ The first pro- 
cess is that within the egg, which represents, so to say, 
the encysted condition of Magosphcera,; the yolk di- 
vides itself inte two balls (Pl. 6, Fig. 6), then into four 
(Pl. 6, Fig. 7), eight, and so on, the cells thus constituted 
I have myself observed 
the same stages in the eggs of the very remarkable and 
abnormal Spherularia bombis§ 
Among the Echinoderms M. Derbés thus describes the 
first stages (Pl. 6, Figs. 1o—13) in the development of the 
egg of an Echinus (Zchinus esculentus) :—*Le jaune, 
commence a se segmenter, d’abord en deux, puis en quatre 
et ainsi de suite, chacune des nouvelles cellules se parta- 
geant a son tour en deux.”|| Sars has observed the same 
thing in the starfish.{) 
* See, for instance, Kauffmann, Uber die Entwickelung und systematische 
stellung der Tardigraden. Zeits. f. Wiss. Zool. 1851, p. 220. 
+ Monograph of the Gymnoblastic or Tubularian Hydroids, by G. J, 
Allman. Ray. Soc. 1871, p. 86. 
t V. Beneden, Mem. sur les Vers Intestinaux, 1858. 
§ Natural History Review, 1861, p. 44. 
ll Derbés, Ann. des Sci. Nat. 1847, p. go. 
‘1 Fauna Littoralis Norvegiss, pl. viii. 
