FORM AND GROWTH 115 



(Jackson 23 ).* Although subject to certain exceptions and limi- 

 tations, this law will be found to apply in general to the growth 

 and development of all vertebrates. 



A slightly older human embryo (Figure 61C), 2. 11 mm. in 

 length, shows the next stage, with precocious development of 

 the central nervous system. By more rapid growth, the sides 

 of the ectodermal neural groove enlarge as folds, which arch 

 over and finally fuse in the midline to form the neural tube 

 (nt). This tube later becomes the brain and spinal cord. The 

 closure of the neural tube begins, not (as might be expected 

 from the law of developmental direction) at the front end, but 

 just behind the head, at the junction with the neck. Accordingly 

 in the stage shown the neural groove is still open in the brain 

 region (nt') y though it closes soon afterward. It still remains 

 open also behind the neck {nt"), progressively closing later to 

 form the spinal cord in the trunk region. 



On each side of the neural tube in the cervical region of this 

 embryo are also seen eight rectangular areas, the somites, or 

 body segments (s). These structures in the mesoderm (middle 

 germ layer) later form the skeleton, musculature, and other 

 structures of the body wall. They appear first in the dorsal re- 

 gion, later extending around the sides into the ventral regions, 

 with the progressive formation of the body wall. These somites 

 are now seen in the neck, but later additional body segments will 

 form similarly throughout the trunk region. 



* As early as the fifth century B.C., the Greek anatomist, Alcmaeon, 1 noted that 

 the head is the first part of the embryo to appear. The disproportionately large 

 size of the embryonic head region has since been observed by many embryologists. 

 Von Baer 3 stated that in a chick embryo of the fourth day: "An Lange betragen 

 Kopf und Hals zusammen ungefahr so viel als der Rumpf. An Masse kommt aber 

 der Kopf allein dem Rumpfe gleich." He later made similar observations on pig 

 and human embryos. His 15 " 19 made numerous measurements establishing this prin- 

 ciple in the chick, human, and other embryos. Jackson 20 ' 21 studied the question in 

 relation to human development topography. Kingsbury 30 has recently reviewed the 

 subject. It is of interest that cephalocaudal progression in development is char- 

 acteristic, not only for vertebrates, but also for many invertebrates, especially the 

 annelids and arthropoda. 



lu I L I B R .' SY- 3 



